photos: rename person to be a contact

In my photos app, I have thousands of photos with a face labelled, say, Joe. I have a few others that seem to be labelled joe@foo.bar, associated with my contact for Joe. I want all my photos all associated with that contact, though truthfully I don't know why. I guess it'd be good in case I ever meet another Joe. Actually, I realize even if I wanted to rename Joe to Joseph, I wouldn't know how to do that either. Help anybody? Not crucial, but this should be obvious and easy and I'm just not seeing it.