How to install the watchOS 3 betaInstalling the watchOS 3 beta is more complex than an iOS or macOS beta, but it's also for developers only.
There's yet to be a public beta for watchOS the way there has been for iOS and OS X, and that's okay — the process for installing the software is more complicated, and really meant for developers only. You need to install a certificate on your Apple Watch, then download the software to your iPhone running iOS 10 beta, transfer it across, and get it up and running. It's not hard, but it does take patience and you may have to troubleshoot.
Apple's 2016 software updates — iOS 10, watchOS 3, tvOS 10, and macOS Sierra — are currently available only as closed developer previews. While the betas contain new features, they also contain pre-release bugs that can prevent the normal use of your iPhone, iPad, Apple Watch, Apple TV, or Mac, and are not intended for everyday use on a primary device. That's why we strongly recommend you stay away, at least until the public iOS and m...
Source:
How to install the watchOS 3 beta
