Apple seeded tvOS 27 RC, build 24J360, to developers and public beta testers yesterday, and the update is now available through Software Update on enrolled compatible Apple TVs (MacRumors, yesterday; iClarified, yesterday). If you're trying to figure out how to install tvOS 27 RC, the requirement comes first: it runs on Apple TV 4K (2nd generation, 2021) or newer, and it does not run on the Apple TV HD (2015) or the original Apple TV 4K (2017) (iThinkDiff, 2 weeks ago; iClarified, yesterday).