Apple has today seeded the fourth beta of iOS 10.2.1 to developers. This comes only a few days after the third release. This likely means that there is a critical bug fix. In this version of iOS we expect to see some fixes and improvements. There are no major UI changes.
To get the latest version, developers can go to Settings and find it under software updates. If you aren’t on the beta yet, you can download the testing profile from the Apple Developer Center.