Following today’s rollout of macOS 10.12.3 beta 4, Apple today seeded both developers and public beta testers with the fourth build of iOS 10.2.1 for debugging before an initial release to the public likely sometime this month. This marks the second beta to be released this month as the third build became available on Monday.
To install the latest iOS 10.2.1 beta on your registered device, head over to Settings > General > Software Update. Tap “Download and install” next to the iOS 10.2.1 Beta 4 listing and you’ll be good to go. I recommend jumping on a Wi-Fi network first alongside plugging in your device beforehand as a) you don’t want data overcharges on your bill and b) you don’t want your phone or tablet to die during the installation process.