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 features will be introduced in the watchOS 5.1.2 update, but as a 5.1.x update, it's likely to be minor in scale, addressing bug fixes and making other small improvements to the operating system. We'll update this post if we find anything new in the watchOS 5.1.2 beta.
watchOS 5.1 and watchOS 5.1.1, the current release version of watchOS, 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.
What's new in watchOS 5.1.2: 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, Phone, Find My Friends, and Remote.
watchOS 5.1.2 also introduces a new toggle in Control Center for turning the Walkie-Talkie feature on and off.