To edit the values presented for selection in a Custom Field, do the following:

1. Go to Contacts > Database Management > Field Management > Manage Custom Fields
2. Expand the folder for either the Custom Profile Fields or Custom Event Fields
3. Find the Custom Field in question in the list and click the Edit link to the left of it
4. On the next screen you will be able to edit the list of items or text that is associated with the custom field you selected
5. At the bottom click the Save Button

NOTE:  The field's data type cannot be altered.  If you need to change the data type, you must create a new custom field.