Registered developers can download the tvOS 11 update by connecting the Apple TV to a computer with a USB-C cable and installing a beta profile using iTunes. Once the profile is in place, new beta updates are available over the air.
tvOS 11 is a modest update that has not received a lot of attention from Apple. It brings automatic switching between light/dark mode based on local time, Home screen syncing options that keep multiple Apple TVs in a household in sync, new background modes and notification support, Focus API improvements, custom sound support, network-based pairing and development support, improvements to Mobile Device Management, and other small changes.
Update: The GM version is also available to members of Apple's public beta program.