Registered developers can download tvOS 11 by connecting the Apple TV to a computer with a USB-C cable and installing the beta software using iTunes.
tvOS 11 didn't receive much attention at the Worldwide Developers Conference because of time constraints, but according to Apple's release notes, it introduces a few new features to the operating system.
tvOS 11 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 more.
The sixth beta of tvOS 11 most likely focuses primarily on bug fixes and other small refinements, as the first five betas did. Apple's new tvOS 11 update is available for both registered developers and public beta testers. It will see a public release later this year.
Update: The fifth public beta of tvOS 11 is now available for public beta testers.