Apple stopped signing iOS 9.2.1, which means you can’t downgrade your iOS version from 9.3.1 to this version. What does this exactly mean? If you try to downgrade to 9.2.1, you will get an error. Also if you try to restore your device running 9.2.1, you will be forced to upgrade. There is no rumor about a jailbreak for 9.2.1 , therefore it should not matter much that Apple stopped signing this iOS version. As a matter of fact, iOS 9.3.1 has a few patched bugs and is very likely to be jailbroken. I recommend that you should upgrade to iOS 9.3.1 if you don’t care about having a jailbroken device or you have an iOS version that does not allow a jailbreak.