If the group has not been checked in yet:
  1. Open the group sale reservation record
  2. In the Manage Itineraries section, click on the arrows next to the itinerary
  3. Click Delete
  4. To recreate the itinerary, click Add
  5. You can now select the correct group type for the reservation

If the group has been checked in, we will not be able to edit the Group Type field. Please review What information can't I change on a Group Sales reservation after I check it in?