The update file size of the 42mm model is pretty big at 515MB and comes with several new features such as new watch faces, Nightstand Mode, Siri improvements and Time Travel with the Digital Crown. The Watch also gets the ability to play video directly although the small screen limits the usability of this feature.
If you have an Apple Watch, go to your iPhone and open the Watch app > General > Software Update. To install the update, you have to ensure the Apple Watch is connected to its charger with at least 50% battery and in range of your iPhone connected to Wi-Fi.