Now you can only downgrade back to iOS 11.2.5 for Apple is no longer signing iOS 11.2, 11.1, 11.0.3, 11 or older. However, this article is still the way to downgrade iOS X.X.X Beta back to a previous version. The steps are the same.
iOS 9.3, released by Apple Inc, is the latest operating system for iPhone and iPad and iPod touch, with some new features such as Night Shift mode and Touch ID security for Notes. Many users who have upgraded to iOS 9.3 on their iPhone said that it doesn't work well due to many bugs and crashes. If you feel that iOS 9.3 is not good for you for whatever reason, you can revert back to the iOS 8.4.1 by following steps. The method below can be applied to iPhone SE/6s Plus/6s/6 Plus/6/5s/5c/5/4s, new smaller iPad Pro and iPod touch with iOS 9.3.
Caution: Before Downgrade iOS 9.3 to iOS 8.4.1
However, there is one thing you should keep in mind first. Once you go back to iOS 8.4.1 from iOS 9.3, you can't restore an iTunes backup from a new version of iOS to an older version of iOS, so you may lose your data by doing this. An efficient way to solve this problem is using iPhone Data Recovery to help you recover your lost files (Photo, Message, Video, Contact, App, Call History, etc).
Steps to Go Back to iOS 8.4.1 from iOS 9.3
1.Open the "Setting" app in your iPhone/iPad/iPod touch and go to iCloud, then turn off "Find My iPhone".
2.Download the iOS 8.4.1 IPSW firmware file for your device. Connect your iPhone/iPad/iPod touch to PC via a USB cable and launch iTunes.
3.Hold down the Power and the Touch ID/Home buttons both at a time for about 10 seconds, then continue holding the Touch ID/Home button while releasing the Power button until you entering the recovery mode.
Tips: iOS system downgrade is subject to stuck on recovery mode. If it happens to you unluckily, try Any iOS System Repair (ReiBoot) to get your iDevice out of recovery mode with one click.
4.Hold the Option key on the Mac or Shift key in Windows and click on "Restore iPhone" option.
5.Select the iOS 8.4.1 IPSW files you have downloaded beforehand and iTunes will inform that it will update your iOS device to iOS 8.4.1. The process may take some time, once it is completed, the device will reboot and you can see the Hello welcome screen of the iOS 8.4.1.
Now, you have downgraded iOS 9.3 to iOS 8.4.1 successfully. Please note that Apple can stop signing iOS 8.4.1 firmware files at any moment, you won't be able to downgrade back to iOS 8.4.1 from iOS 9.3 by then. So please downgrade as soon as possible. If you change your mind, you can always upgrade back to iOS 9.3 again.