Apple released tvOS 10 today, which is available for the 2015 model of the Apple TV.
The update begins to refine the interface and round out the features that Apple introduced on the new Apple TV last October. That includes updates to Siri, which can now search YouTube and control HomeKit products, and the ability to replace your Apple TV remote with a remote app on the iPhone or iPad. There's now also a dark mode, which might look way better on your TV than the bright interface the system has been using since last year.
Unfortunately, tvOS 10's biggest new feature is still forthcoming. A single sign-on feature will allow owners to authenticate their cable TV login once and have it applied to every single app that uses it. For some reason, that's been held back till later this year.