How to remove a photo from iCloud people albums
I was recently organising my photo library on iCloud when I noticed I had incorrectly confirmed a photo of my son to my daughter's album.
It was a bit tricky to fix this so I thought I might share the process.
Select your person from the "People" album, DO NOT start selecting photos yet, tap the "Select" button in the upper right-hand corner.
Tap on the misconfirmed photo, tap the "Share" icon in the bottom left, then from the list at the bottom of the screen select "Not This Person."
If you need to select multiple photos, check all applicable pictures from the scrollable list at the top of the Share sheet prior to pressing "Not This Person."