You can add/remove workflows to the list or library by navigating to

List Settings -> Workflow Settings -> Add a workflow

List Settings -> Workflow Settings -> Remove a workflow

If you want to perform the same operation using PowerShell script, here is the script to add a workflow to a list

Script to Add Workflow

  param(

  [string] $env = $(Read-Host -prompt "Specify the environment you want to run on (D-Dev, T-Test, P-Production) :") 

)

 

$env = $env.ToLower()

 

if($env -ne 'd' -and $env -ne 't' -and $env -ne 'p' -and $env -ne 'l') {

    Write-Host "ERROR: Invalid Input."

}

else {

 

                $siteAddress = $null

 

    if($env -eq 't') { $siteAddress = 'http://test.yoursite.com/' }

    elseif($env -eq 'd') { $siteAddress = 'http://dev.yoursite.com/' }

                elseif($env -eq 'p') { $siteAddress = 'http://yoursite.com/' }

   

 

echo $siteAddress

 

  # Get the SPSite object for a given SharePoint Url

  $SPSite = Get-SPSite $siteAddress

  if($SPSite -ne $null)

  {

                # Get the root web site

    $Web = $SPSite.OpenWeb();

 

                # Get the list to which we wanted to associate the workflow

                $SPList =$Web.Lists["Announcements"];

 

                # Get the Approval Workflow Template by specifying current culture info

                # $Culture = New-Object System.Globalization.CultureInfo("en-US");

                $Template = $Web.WorkflowTemplates.GetTemplateByName("Approval - SharePoint 2010",[System.Threading.Thread]::CurrentThread.CurrentCulture);

 

                # Try to get workflow history and task list 

                $TaskList = $Web.Lists["Workflow Tasks"];

                $HistoryList = $Web.Lists["Workflow History"];

 

    # Create workflow history list if it doesn't exist by default.

    if($HistoryList -eq $null)

    {

                   $Web.Lists.Add("Workflow History", "My Workflow History","Lists/Workflow History", "00BFEA71-4EA5-48D4-A4AD-305DE7030140", 140, "101");

                   $HistoryList = $Web.Lists["Workflow History"];

    }

 

                # Create the Workflow association by using Workflow Template, Task List and History List

                $Association=[Microsoft.SharePoint.Workflow.SPWorkflowAssociation]::CreateListAssociation($Template, "SharePoint Approval Workflow", $TaskList, $HistoryList);

 

                # Enable automatic workflow start when news created or changed and disable manual start option

                $Association.AllowManual = $false;

                $Association.AutoStartChange = $true;

                $Association.AutoStartCreate = $true;

 

                # Provide Association Data which includes as below

                #             1. Approvers SharePoint group for list of approvers.

                #             2. Serial order Workflow.

                #             3. Specify the notification message while sending notifications to approvers.

                #             4. Automatically reject the document if it is rejected by any participant.

                #             5. Update the approval status after the workflow is completed (use this workflow to control content approval).

                $approvalAssociationData = "

                <dfs:myFields xmlns:xsd='http://www.w3.org/2001/XMLSchema'

                                xmlns:dms='http://schemas.microsoft.com/office/2009/documentManagement/types'

                                xmlns:dfs='http://schemas.microsoft.com/office/infopath/2003/dataFormSolution'

                                xmlns:q='http://schemas.microsoft.com/office/infopath/2009/WSSList/queryFields'

                                xmlns:d='http://schemas.microsoft.com/office/infopath/2009/WSSList/dataFields'

                                xmlns:ma='http://schemas.microsoft.com/office/2009/metadata/properties/metaAttributes'

                                xmlns:pc='http://schemas.microsoft.com/office/infopath/2007/PartnerControls'

                                xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>

                                <dfs:queryFields></dfs:queryFields>

                                <dfs:dataFields>

                                                <d:SharePointListItem_RW>

                                                                <d:Approvers>

                                                                                <d:Assignment>

                                                                                                <d:Assignee>

                                                                                                                <pc:Person>

                                                                                                                                <pc:DisplayName>Approvers</pc:DisplayName>

                                                                                                                                <pc:AccountId>Approvers</pc:AccountId>

                                                                                                                                <pc:AccountType>SharePointGroup</pc:AccountType>

                                                                                                                </pc:Person>

                                                                                                </d:Assignee>

                                                                                                <d:Stage xsi:nil='true' />

                                                                                                <d:AssignmentType>Serial</d:AssignmentType>

                                                                                </d:Assignment>

                                                                </d:Approvers>

                                                                <d:ExpandGroups>true</d:ExpandGroups>

                                                                <d:NotificationMessage>Please have a look at submitted news.</d:NotificationMessage>

                                                                <d:DueDateforAllTasks xsi:nil='true' />

                                                                <d:DurationforSerialTasks xsi:nil='true' />

                                                                <d:DurationUnits>Day</d:DurationUnits><d:CC />

                                                                <d:CancelonRejection>true</d:CancelonRejection>

                                                                <d:CancelonChange>false</d:CancelonChange>

                                                                <d:EnableContentApproval>true</d:EnableContentApproval>

                                                </d:SharePointListItem_RW>

                                </dfs:dataFields>

                </dfs:myFields>";

               

                $Association.AssociationData = $approvalAssociationData

 

                # Associate the approval workflow to News List

                $Workflow = $SPList.WorkflowAssociations.Add($Association);

                if($SPSite -ne $null)

                {

                                Write-Host "Workflow has been attached successfully." -foregroundcolor "Green";

                                $SPList.Update();

                }

                else

                {

                                Write-Host "Workflow could not be attached." -foregroundcolor "Yellow";         

                }

  }

}

 

Script to Remove Workflow

  param(

  [string] $env = $(Read-Host
  -prompt "Specify the environment you want to run on (D-Dev, T-Test,
  P-Production) :")

)

 

$env = $env.ToLower()

 

if($env -ne 'd' -and $env -ne 't' -and $env -ne 'p') {

    Write-Host "ERROR:
  Invalid Input."

}

else {

 

                $siteAddress =
  $null

 

    if($env -eq 't') {
  $siteAddress = 'http://test.yoursite.com/' }

    elseif($env -eq 'd') {
  $siteAddress = 'http://dev.yoursite.com/' }

                elseif($env -eq
  'p') { $siteAddress = 'http://yoursite.com/' }   

 

 

  # Get the SPSite object for a
  given SharePoint Url

  $SPSite = Get-SPSite
  $siteAddress

  if($SPSite -ne $null)

  {

                # Get the root
  web site

    $Web = $SPSite.OpenWeb();

 

                # Get the list  to which we wanted to associate the workflow

                $SPList  =$Web.Lists["Announcements"];

 

                $workflowAssociation  = $SPList.WorkflowAssociations.GetAssociationByName("SharePoint Approval
  Workflow", [System.Threading.Thread]::CurrentThread.CurrentCulture);

 

                if($workflowAssociation  -ne $null)

                {

                                #  Remove workflow association to list

                                $SPList.RemoveWorkflowAssociation($workflowAssociation);

                                $SPList.Update();

                                Write-Host  "Workflow association removed Successfully." -foregroundcolor  "Green"

                }

                else

                {

                                Write-Host  "Workflow association could not be found." -foregroundcolor  "Yellow"

                }

  }

}