Here is the sample code,

                SPWeb web = new SPSite("http://karthick64:4040/").OpenWeb();
                SPField spField = web.Fields["MyChoice"];
                SPFieldChoice choiceFields = (SPFieldChoice)spField;
                string[] choices = new string[3] {"x", "y", "z"};
                foreach (string choice in choices)
                {
                    choiceFields.Choices.Add(choice);
                }
                choiceFields.Update();

Keep Exploring... :)