Basically, those who are interested in picking up iOS 9.2.1 Public Beta 1 will be able to do so if you have previously signed up for Apple’s beta testing program. Should you be in one of these coveted groups, so to speak, then it should not be too long before your eyes are greeted by an update notification.
It remains to be seen as to what resides within the new software, simply because there is no changelog to refer to at the moment. Taking into consideration that this is a 9.2.1 release, one should not have too high expectations on new features, apart from the now usual slew of bug fixes and performance enhancements. Fingers crossed, should the beta testing go well, chances are the finalized version of iOS 9.2.1 should be released in due time.