The tvOS 10.2.1 beta is designed for the fourth-generation Apple TV. It can be downloaded by connecting the Apple TV to a computer with a USB-C cable and installing the beta software from a registered developer account using iTunes.
Because of the tricky installation requirements, tvOS betas are limited to developers. tvOS 10.2.1 will not be available to the public until the final version of the software launches.
Apple does not typically provide detailed beta release notes, so we don't yet know what's included in tvOS 10.2.1. It's likely to focus primarily on bug fixes and under-the-hood performance improvements, so we may not know what's new until the software sees an official release.
Should any noticeable changes be discovered in tvOS 10.2.1, we'll update this post.