Apple has released watchOS 4.2 for Apple Watch with Apple Pay Cash support.
watchOS 4.2 can be downloaded through the dedicated Apple Watch app on the iPhone by going to General –> Software Update. To install the new software, the Apple Watch needs to have at least 50 percent battery, it needs to be placed on a charger, and it needs to be in range of the iPhone.
After setting up Apple Pay Cash on iPhone with iOS 11.2, Apple Pay Cash becomes an option in watchOS 4.2 in the Messages app and Wallet app on Apple Watch. Apple Pay Cash also works with Siri.
In Messages, Apple Pay Cash is a new quick action below Scribble that you see when replying to an iMessage. This lets you send up to $3,000 to friends with Apple Pay Cash. In Wallet, Apple Pay Cash lets you pay in stores and in apps on Apple Watch using the service’s virtual debit card. Siri can be used to both send and request payments with contacts.