Beta testers who have signed up for Apple's beta testing program will receive the new iOS 10.2.1 beta update over-the-air after installing the proper certificate on their iOS device.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to both iOS and macOS Sierra betas. Betas are not stable and include many bugs, so they should be installed on a secondary device.
It isn't known what features are included in iOS 10.2.1, but as a minor 10.2.x update, it appears to focus on bug fixes and performance improvements rather than major outward-facing changes.
No new features were discovered in the first two developer iOS 10.2.1 betas, and we likely won't know the full extent of the changes until the update is released with full release notes.