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.
As was discovered in the first beta, the watchOS 5.1.2 update introduces new complications for the two Infograph watch faces on the Apple Watch Series 4. New complications include Mail, Messages, Home, Maps, Apple News, Find My Friends, Phone, and Remote.
watchOS 5.1.2 also introduces a new toggle in Control Center for turning the Walkie-Talkie feature on and off.
The previous watchOS 5.1 and watchOS 5.1 updates introduced support for Group FaceTime in an audio-only capacity, brought new emoji characters, and included a new full-screen "Color" watch face option for the Apple Watch Series 4 models.