Apple has four Operating systems namely iOS, macOS, tvOS and watchOS. If you have an Apple watchOS, then you can use the latest developer beta. If you want to use Beta 2 in your Apple watch, then first, you have to install a certificate into your watchOS. After that, you need to download software in your current iPhone device to transfer it and vice versa.
Please read the originial post here: https://harispot.com/how-to-install-watchos-6-1-2-beta-2-on-apple-watch/