Set default image for a person in Photos There is one question that has an answer that seems to mention a process for this in a previous version of Photos, but I'm not seeing how to do this in Photos now (version 5.0 on macOS and whatever it is on iOS 13.2)
Go to people tab and open the person you're wanting to change the key photo of. Find two tabs "Photos" and "Faces".


  


In Faces, find the face you want, right click and in the end, choose "Make Key photo".