Apple has
stopped signing iOS 9.2.1 for all the iOS devices like iPhone, iPad
and iPod Touch which means that users can no longer downgrade their
iOS to that version using iTunes. Now you can’t downgrade to iOS 9.2.1,
If you are on iOS 9.3.1/9.3 and also cannot upgrade to iOS
9.2.1, if you are on lower versions. So basically you are forced to upgrade to
iOS 9.3.1 directly.
Apple has stopped signing iOS 9.2.1 for all the iOS devices like iPhone, iPad and iPod Touch which means that users can no longer downgrade
Apple
always gives two versions of iOS to restore whenever it releases a new
version. So that people have ability to downgrade to the previous
version if they are not satisfied with the new one. But after some time, it
stops signing the previous version.

Note: If
you are on lower versions like 9.1, 9.1.2, 9.2, 9.2.1, do not upgrade to
iOS 9.3.1/9.3 because no one knows for what version of iOS there will be a
jailbreak. We all are waiting for jailbreak to be released. So it’s better to
stay on older version of iOS for now.