tvOS 12.1 can be downloaded over the air through the Settings app on the Apple TV by going to System -> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to tvOS 12.1 automatically.
It's not clear what new features or bug fixes are included in the tvOS 12.1 update. We didn't find any major new changes during the tvOS 12.1 beta testing period, and Apple doesn't provide release notes for tvOS releases, so there's not often a lot to go on.
If we discover what's new in tvOS 12.1, we'll update this post with additional information.
tvOS 12.1 follows the tvOS 12 update, which introduced support for higher-quality Dolby Atmos sound, zero sign-on, and new aerial screensavers created in collaboration with the International Space Station.