I already reset my iPhone and watch twice and it keeps saying I gotta update to the latest software, but the latest is 8.4 and I have 8.4 on my iPhone.
Ahhh - you bought this used it appears. I honestly can't understand what would be going on. Really makes no sense so I hope someone offers a good solution.
What I would do at this point is call Apple. Otherwise you do a backup of your iPhone and wipe/restore