iOS Downgrade is signing your device back to Apple previous version. There are several reasons for the downgrade. At this time, you cannot downgrade to any iOS version. So now you can downgrade iOS 9.3.3 to iOS 9.3.2 only.
Go to this site and download your firmware version according to your device model.
Disable Touch ID and Passcode Settings > Touch ID & Passcode > Turn off Passcode
Disable Find my iPhone Settings > iCloud > find my iPhone – turn off
Download latest iTunes. If you already downloaded check for update
Connect device to PC and open iTunes.
Windows users should use Shift and iTunes restore
Mac users should use Option and iTunes restore.
Now select previously downloaded firmware file and enter to selection window.
Click Restore button.
Finish, now you have successfully downgraded to the previous version. Now go through settings and setup the device.
Reasons for downgrade
With iOS updates, Apple disabling current jailbreak ability. So you can rejailbreak your device from iOS downgrade.
Apple is always increasing device performances / removing bugs from new iOS updates. However, some iOS users having problems after update to latest version ex- Wi-Fi connection issue / battery drain issue / message issue. So those users can remove these kinds of issues after the downgrade.
Can downgrade after Apple’s stopped iOS sign?
Yes, you still can downgrade back to the previous version even if Apple stopped signing option. However, you should save SHSH blobs before Apple stop signing option. TinyUmbrella / iFaith and Redsn0w are most compatible tools for saving SHSH blobs. After you can downgrade anytime with saved SHSH blobs, even Apple stopped signing.
iOS Downgrade Errors