For technical reasons, the system doesn't enable the modification of auth types for the group already saved.
However, you can achieve a similar result by cloning the Group. When you clone a group, you create a copy of the group with all the users, doors and schedules already assigned. However, you can choose different auth types and, if the auth types/credentials of the old group are no longer valid, delete the old group, ending up with essentially the same group but modified auth types.
Read more about it in the Groups article.