How to delink a device from my iCloud?

Hey all. I've searched the forum and it seems the relevant questions are about removing an iCloud account from your phone. I want to remove an iCloud account from another person's phone. Basically, I gave my old phone to a relative, did not think about it, and now their photos load up to my iCloud. They literally do not understand me (language barrier) so I was wondering if I could delink their device from my account. Thank you!