tvOS 9.2.1 is a minor update that comes more than a month after the launch of tvOS 9.2, a significant update introducing features like Bluetooth keyboard support, dictation, Siri search for the App Store, app folders, and more.
tvOS betas are more difficult to install than beta updates for iOS and OS X. Installing the tvOS beta requires the Apple TV to be connected to a computer with a USB-C to USB-A cable, with the software downloaded and installed via iTunes or Apple Configurator. Those who have already installed the first two tvOS 9.2.1 betas will be able to update to the third beta over the air.
As a minor 9.x.x update, tvOS 9.2.1 appears to focus on bug fixes and performance improvements that are not readily obvious to Apple TV users. No outward-facing changes have been found in the first two tvOS 9.2.1 betas, but should any new features be found in beta 3, we'll list them below.