

Beta testers who have signed up for Apple's beta testing program will receive the iOS 9.2.1 update over-the-air after installing the proper certificates.
Those who want to be a part of Apple's beta testing program can sign up to participate on the company's beta testing website, which gives users access to both iOS and OS X betas.
The first developer beta of iOS 9.2.1 revealed no significant outward-facing changes, suggesting iOS 9.2.1 is a minor update that will focus on under-the-hood performance improvements and bug fixes to address issues discovered since the release of iOS 9.2.
Article Link: Apple Releases First iOS 9.2.1 Beta for Public Beta Testers