Apple has released watchOS 4.2 for the Apple Watch. It adds support for Apple Pay Cash service, as well as other improvements and bug fixes.
You can install watchOS 4.2 via the dedicated Apple Watch app on the iPhone by going to General --> Software Update. Your smartwatch needs to have at least 50% battery, be placed on a charger, and it needs to be in range of the iPhone.
With Apple Pay Cash, Apple Pay users will be able to make and receive payments with friends and family quickly and securely. They can send money and get paid right in Messages, or tell Siri to pay someone, using the credit and debit cards they already have in Wallet.