How to Downgrade iOS 9.3 to iOS 9.2.1
The newly released, iOS 9.3, it offers a lot of very nice features. One of them is the Night Shift feature. iOS 9.3 is a refinement of iOS 9.2.1 so it has better performance.
However, if you has already upgrade your device into iOS 9.3 and want to restore your device to iOS 9.2.1 (note, in this way you can do as long as Apple has not closed the firmware). You can do it using this way.
You can use to your iPhone, iPad, and iPad Touch via iTunes on the Mac or PC, as reported by Osxdaily.com.
First of all, you need to backup your iOS, this is an important thing that you should do. This way you can do via iTunes also iCloud and (iOS Backups is an important thing do not cancel or miss it).
Once the backup is complete, you can download iOS 9.2.1 IPSW firmware file that matches with your iOS device. This file will not work if it does not match with the device type. You can see the list at the link above.
File IPSW