The signing for iOS 9.3.1 for all iPhones,iPads,iPods got stopped by Apple!
This means that updating to iOS 9.3.1 is not possible anymore nor you can’t downgrade to older versions! In principle this won’t likely be relevant for any jailbreak interested parties. Because the Pangu had his attention mostly one iOS 9.3.3 and iOS 10 and they even have already presented a working Jailbreak for iOS 9.3.3 at the MOSEC – security conference.
If you have questions, just wright at the comment section below!