Here’s what the release notes say is in the update:
Apple has a document detailing watchOS 2.2’s security content.
Installation requires iOS 9.3 on your iPhone. When installing a watch update, your watch has to be in range of your iPhone, connected to its charger, and charged to 50 percent of battery life.
Once your hardware is all set up, open the Watch app on your iPhone. Tap General > Software Update. The update is 114MB and will take several minutes.