The iOS 9.2.1 first beta for developers is available for those registered Apple Developers. Apple didn't state out any release note information, so this just happens to be yet another weekly round of software update to the developers, more importantly increment in iOS version. It will bring out stability, compatibility, bug fixes and performance improvements.
The iOS 9.2.1 beta 1 developer only update is compatible with -
- iPhone 6S, iPhone 6S Plus, iPhone 6, iPhone 6 Plus, iPhone 5S, iPhone 5C, iPhone 5, iPhone 4S
- iPad Air 2, iPad 4, iPad Air, iPad Mini, iPad 3, iPad 2
- iPad Mini 3, iPad Mini with Retina display, iPad Mini
- 5th and 6th generation of iPod Touch
Both the software updates are available via Over the Air download normally