The update comes as an over the air software download for tester’s iOS devices enrolled in the program. Users can download the new build by going to Settings > General > Software Update.
If you’d like to enroll in the public beta program and test out the next version of iOS, visit the official public beta program website by clicking here and follow the instructions listed there to enroll your iOS device.
Mac Rumors has explained what you can find in the new beta:
iOS 9.2 fixes several bugs in the operating system and it includes other under-the-hood enhancements. According to the release notes for the first beta, the Safari View Controller (the view of Safari you see within apps) is being updated with support for third-party Action Extensions that will let it more closely mirror the Safari app. It also includes options to reload content and request a desktop site, as in Safari