Code from fb 2.5 does not work:

update rdb$relation_fields set rdb$description = 'aaa'
where rdb$field_name = 'new_name" and rdb$relation_name = 'table_name'

Update operation is not allowed for system table rdb$ralation_fields

FB 3.0.2.32692

--
pozdr,
Michał Kurczabiński