Hi, so if you want to change the default photo of a person shown in the Persons folder in photos you have to follow following steps:
1. Open photos
2. Tap on Albums
3. Go to the People Album
4. Tap on the person you which to change the default photo
5. Tap "Select" in the right upper corner
6. Select the photo you want to have as default photo
7. Tap on the share button
8. Select "Make Key Photo"
9. Done!
Good luck!
Greetings, lb