Once the proper configuration profile has been installed from the Apple Developer Center, the new watchOS 4.2 beta can be downloaded through the dedicated Apple Watch app on the iPhone by going to General --> Software Update.
To install the update, the Apple Watch needs to have at least 50 percent battery, it must be placed on the charger, and it has to be in range of the iPhone.
watchOS 4.2 brings support for Apple Pay Cash, Apple's new peer-to-peer payment service that is designed to let users send money to one another. Apple Pay Cash can be used to send and receive money through the Messages app on the Apple Watch.
Aside from Apple Pay Cash, watchOS 4.2 appears to focus primarily on under-the-hood bug fixes and security updates.