tvOS 11 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 11 automatically.
The tvOS 11 update introduces full support for AirPods, AirPlay 2 support, automatic switching between light and dark mode based on local time, Home screen syncing options designed to keep multiple Apple TVs in a single household in sync, and new background modes and notification support.
On the more technical side, the update includes Focus API improvements, custom sound support, network-based pairing and development support, improvements to Mobile Device Management, and more.
For the fifth-generation Apple TV, aka the Apple TV 4K, the tvOS 11 update introduces support for 4K content. Apple's new 4K Apple TV will ship with tvOS 11 installed by default.
Additional improvements to the TV app for Apple TV and iOS devices will bring support for live news and live sports following the release of tvOS 11 and iOS 11, and later this fall, tvOS 11 will gain support for the Amazon Prime Video app.
tvOS 11 is limited to the fourth and fifth-generation Apple TV models and is not available on earlier models.