Edit the definition of an existing custom field. field_type and value_type are immutable — to change either, create a new field. As with create, the accepted properties vary by the field's existing field_type: job and opening fields expose approval and visibility toggles, offer fields expose trigger_new_version, user attributes are limited to name, description, and required. Manage option lists for select-type fields through /v3/custom_field_options, not this endpoint.