Apple has released a minor update to tvOS today for the forth-generation Apple TV HD and Apple TV 4K. tvOS 12.2.1, which is based on iOS, was released today bringing minor bug fixes and other improvements to the platform. This release is purely a bug fix update.
There are no new features to be found in this release of tvOS so don’t expect to see anything new. I point this out as there may be confusion with the expected new Apple TV app makeover. That makeover is expected in 12.3, which is currently in beta. It is likely 12.3 won’t be released until mid to late May base don how beta cycles go on the build. That will coincide with the iOS 12.3 update which brings a new look to the Apple TV app on that platform too.
Your Apple TV device should automatically check for and prompt you to install the update. However, you can force the issue by going into Settings>General and checking for the software update there. It is a small update so shouldn’t take more than a few minutes to install.
As for why Apple felt compelled to released a minor dot release is not clear. The release notes for the build only cite bug fixes but do not get into the specifics of what exactly what was addressed. Clearly however it was enough for them to drop an unexpected update.