Once the proper configuration profile has been installed from the Apple Developer Center, the new watchOS 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.
It's not yet clear what new features or improvements the watchOS 4.3 beta might introduce, but we'll update this post once we've installed the update and have had time to look for any new features.
Update: watchOS 4.3 introduces support for Nightstand mode when in portrait orientation, it includes a new charging animation when placing the Apple Watch on the charger, and it adds Activity information to the Siri watch face.
The update also includes the return of a much-desired feature that allows music playing on the iPhone to be controlled using the Music app on the Apple Watch. In prior versions of watchOS, the Music app could only be used to control music playing from the watch itself.