Apple has today released a lot of software, included in that spurge of updates is a minor one for the Apple TV HD and Apple TV 4K. tvOS 15.1 includes support for SharePlay, to go along with the added support on iOS 15, macOS Monterey and iPadOS 15.
tvOS usually updates automatically, but if you want to manually upgrade, it can be done so by going to Settings > System > Software Update.
