To go along with the fresh iOS 10.2.1 and macOS Sierra 10.12.3 updates today, Apple also rolled out updates for tvOS and watchOS. watchOS 3.1.3 is the first update since 3.1.1 hit the streets back in December -- I could never personally get watchOS 3.1.1 to install on my Watch, so it will be interesting to see if the update I'm doing right now succeeds.
The watchOS update can only be installed through the Watch app on the iPhone, and is found by tapping on General > Software Update. You'll need to have your iPhone nearby throughout the update, and the Watch must be plugged into a charger and have at least 50% battery life remaining.
tvOS 10.1.1 looks like it's a maintenance update with few differences from 10.1, which was released in December. If you don't have your fourth-generation Apple TV set up to do automatic updates, you'll need to go into Settings > System > Software Update to download and install the new tvOS.