Currently custom fields are only editable for staff “indirectly” on the users preferences page. The user list page itself (on https:///admin/users/) should have an option to edit a custom field for newly created user accounts. E.g. if the user accidently selected the wrong community, as in my case. Similar to the functionality for “edit badges” etc:
See also Adding and editing user custom fields and Allow staff to change uneditable user fields · discourse/discourse@6d38005 · GitHub