Apple has released iOS 9.2.1 to developers for testing. The build arrives as 13D11 and is compatible with any iPhone, iPad, or iPod touch which an run iOS 9 releases.
At the moment, the iOS 9.2.1 beta build is available through the iOS Developer Center website to download, and presumably the public beta release will be available shortly as well.
It’s unclear what iOS 9.2.1 aims to focus on, but presumably it will be minor bug fixes and improvements to whatever issues are discovered with iOS 9.2, which was just released.
The iOS 9.2 update is the most recently available stable build of the mobile software for iPhone, iPad, and iPod touch users. Multiple reports, anecdotal and otherwise, suggest that iOS 9.2 runs faster than iOS 9.1 did, but still generally slower than iOS 8.4 did on the same hardware.