Registered developers can download the tvOS 11 update 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 a lot of attention at the Worldwide Developers Conference because it's a modest update, but according to Apple's release notes, it does introduce 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 ninth beta of tvOS 11 most likely focuses primarily on bug fixes and other small refinements, as the first eight betas did. Apple's new tvOS 11 update is available for both registered developers and public beta testers and will see a public release this fall.
Update: Apple has also made a new public beta of tvOS 11 available for public beta testers.