The biggest addition to the software is support for SharePlay. That allows you to use FaceTime and watch movies or TV shows with friends and family no matter where they are.
The software will automatically be downloaded to a Apple TV HD or Apple TV 4K.
If you haven’t already enabled automatic updates, you can download the software by heading to Settings > System > Software Updates > Update Software. Right the button to update software is the toggle switch to turn on automatic updates.