1: SPSite spSite = new SPSite("http://localhost");

   2: SPWeb spWeb = spSite.OpenWeb();

   3: SPList spList = spWeb.Lists["Shared Documents"];

   4: SPList workflowlist = spWeb.Lists["NewHistoryLibary"];

   5: SPWorkflowAssociation association = spList.WorkflowAssociations[0];

   6: association.SetHistoryList(workflowlist);

   7: spList.UpdateWorkflowAssociation(association);