iphone 6 plus ios update stuck on update completed screen.

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.

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.