1. Head under Configuration to Custom Fields

2. Locate the field you want to edit.  If you have multiple fields with the same or similar name, use the "Custom Field type" column to make sure you have the correct field.  Click the field's name.

3. Locate the "Manage Selection Values" link near the bottom of the modification box.  Click on "Manage Selection Values".  The number in the square brackets after the link is how many values are currently in that list so may be different than in the screenshot below.

4. In the new screen that pops up you are able to adjust which value is the Default for new users using the Default column, you can delete a value by checking the box under Delete.  You can add a new value with the New Value text box.  No changes, including additions and deletions, will happen until you hit Save

5. Once you are done making your changes, click Save.  You can only enter one New value per save.  You can import values into this field using the Custom Fields Import.