on system administration module, custom fields, picklist type fields values are not editable.

As an example:

We have created a custom field. Search at the top searching bar for “Custom Fields”

Then choose the table “HcmWorker”:

Then choose the custom field. And go to the section “Values”:

There you have the possibility to add new values and to change the order of the values but you cannot delete a value or edit a value

When you click on “Edit” at the top, you still cannot change anything even with System Administrator Role.

Which means:

If you want to change these values (which is the picklist of the field), you have to delete the whole custom field and create a new one, which is not practical in the daily business since this field is connected to several other systems through interfaces and also in Power BIs.