Apple has released the final and stable version of watchOS 4.2 this afternoon, a few days after the unusual launch of iOS 11.2 in the middle of the weekend.
You can now update your watch, no matter the model because watchOS 4 is compatible with all generations.
The main novelty of watchOS 4.2 is the same as iOS 11.2: support for Apple Pay Cash. We can use the Apple Watch and Siri to send money to other people, loudly, although everything is done through iMessage.
This service, remember, is available only in the United States at the moment
Have patience, since everything in the oldest generations of the clock the update can last a good while.