iphone 6 plus ios update stuck on update completed screen.

If the update has completed you should be safe to perform a forced reset (reboot). Press and hold both the side button and the home button until the Apple logo appears on screen, then release the buttons and let the phone restart.

If you got a successful reboot (Apple logo then the progress ring) but it goes back into the same failure mode it seems likely you have a failed update. Contact Support (link) and consult the professionals.