From this article:
Launch the Photos app on your Mac.
Select the picture that has the person's face you want to create a
Click on the Info button on the top right side of the Photos window.
It looks like an i with a circle around it.
Click the Add button next to Add Faces.
Relocate the tag circle to the face of the person you want to create a
Click on Click to Name.
Name the person. If the name is in your contacts, it will appear in
the suggested list.
Click the Close button in the info window when you're done.
Click on the People album in the sidebar under Library.
Select the new People profile.
Click on Confirm Additional Photos to see if the Photos app has
automatically registered any additional pictures with that person in