According to Apple, the brings HD previews for movies and TV shows from the iTunes Store.

The rest of changes are bug fixes: fixing an issue that caused some iOS app to have trouble connecting via AirPlay, improving the reliability of Home Sharing connections, addressing an unspecified issue with Neflix login and navigation, and the obligatory "fixes for issues affecting stability and performance."

You can update the software on your Apple TV by navigating to Settings -> General -> Update Software and following the on-screen directions.