How Do I Change a Field Name In a Contact Group for All Contacts In the Group?

I imported a membership contact list into contacts. However, ‘membership #’ is not a field. I imported it to a standard field like ‘parent name’….but now I want to change ‘parent name’ for all the contacts in the group to ‘membership #’.
I can do this one at a time, but it would be very time consuming.
Is there a way to do this for all contacts in this group.
Thanks in advance, Matt
—–
Matt Balla