Apple leaves the signing window for older versions for usually a week after the newest version drops.
Go to a site like https://ipsw.me/ and find your device. If the iOS version in the drop down menu has a green background, you can still restore to that version. You download the ipsw file and hold shift + click restore/upgrade on iTunes to manually target the ipsw you want to downgrade to.
Thing to note is that I didn't have the bug in 9.2.1 so I assume that something else changes within the upgrade process (i'm no iOS expert plainly) whether it be firmware of whatever, which doesn't get downgraded.
I find it somewhat questionable that I can revert to a previous version and still have that the problem the upgrade caused. I'm guessing this is my lack of understanding about mobile devices though. The whole process made me feel rather impotent compared to troubleshooting a PC or similar.
edit: (I should point out that I had to go to a laptop to post this comment, since the reply link still won't work on the iphone!)
How? I thought Apple didn't allow that. I'd like to go back to 9.1 if possible