Apple has just released tvOS 10 to the public. It has been in development for four months and gone through several betas to developers and public beta testers. tvOS 10 brings things like single sign-on for TV service providers, dark mode, Siri improvements, redesigned Apple Music, and more.
To get this update you will need to go to Settings > System > Software update > tvOS 10. This update will only work on the fourth generation Apple TV. It’s a large file so it may take some time to download and install.