How to downgrade your iOS version
WARNING: run a backup first
1 – On the back of your device, check the model number:
2 – go here: https://ipsw.me/device-finder
3 – choose “iPhone” or “iPad” and enter the model number.
4 – Download the file (big file!) of the version you want to downgrade to. Save it on your desktop.
5 – Disable “find my iphone/ipad” on your device.
(settings > click your name > icloud > find my iphone/ipad > disable)
6 – open iTunes on your computer, and connect your iPhone or iPad, wait until it appears in iTunes in the top left of iTunes
Mac: press and hold Alt/Option-key and click “restore”
Windows: press and hold shift-key and click “restore”
8 – select the downloaded file (IPSW-file)
9 – confirm the reinstall