Roughly a month after the last major Apple Watch software update, Apple has released watchOS 4.2, bringing support for Apple Pay Cash, among other changes.
Those other small changes include a new workout type for third-party apps to track different metrics (distance, speed, elevation descended for downhill snow sports, number of runs), as well as support for HomeKit sprinklers and faucets.
Apple fixed a bug related to Siri where your Apple Watch could restart when you asked about the weather. An alarm/timer dismissal bug, and an issue that disabled scrolling in the Heart Rate app for certain users have also been fixed in this update.
To update your Apple Watch, be sure it is within range of your iPhone, on its charger, and has at least 50% battery life. Then visit the Apple Watch app on your iPhone, then go to General > Software Update.