If a picture of Jack is identified as Jill, then you can just click on the name in the picture and type in Jill. The picture will be moved from the Jack album to the Jill album.
If, instead the picture says Jack but you don't know who it is, you can click on the name and leave it blank-- it will then say "unnamed."
On a Mac, if a picture has an unrecognized face (it doesn't say "unnamed,") you can use stuff in the Info Window (⌘-i) to add the identifier. In the info window, below the Keywords field, is the Face ID field, and in there is a little + sign in a circle:

When you click on that, a little circle appears in the center of the picture, and you can resize it and move it to the face you want to record:

Then you enter the name, like Charley or Prince of Wales, or whatever. Then, if Charley has no People album, one will be created and this picture will go there. Later, if you need to change the name for some reason, you can do that in the People album, maybe making it King Charley III, or something.
The one problem we sometimes have is that the circle refuses to move, because Photos thinks you're still in the info window and wants to drag the whole picture there, or something like that. In that case you need to tap on the picture to let Photos know you mean the picture, and then you can move the circle. Or, you can just type the name first, and then move the circle. This seems to me to happen less in Sequoia.
You should know that manually naming a face doesn't affect how Photos chooses faces and identifies them. When you change "unnamed " to Charley, it helps Photos understand what a "Charley" face is. But manually adding a Charley circle only adds his face to his album.
Does this work for you?