Custom Data and Screens
The only way to designate an option group as reserved is to do so directly through the database - there is no option to do this through the UI either when it is created via a multiple-choice custom data field or when added directly via Administer > System Settings > Option Groups - there might be a reason that this option group was flagged as reserved, so you might wish to ask whoever is responsible for administering CiviCRM to shed light on the reason and/or ask them to remove the reserved flag altogether so that you can make changes when needed.

Hope this helps,
Tamar