Apple’s new watchOS 8 is now released in public beta, but it has one important caveat. Once installed on your Apple Watch, there is no way to recover the previous OS. Unlike in IOS 15, you cannot restore your previous OS, so if you intend on beta testing watchOS 8, note that you may run into unavoidable bugs and issues that will remain until they are fixed.
The watchOS public beta updates are still in development, which means some applications and services may not work as expected, and their data may not be backwards compatible. Apple Watch content backs up automatically to your paired iPhone. You can learn more about Apple Watch backups here. Please note Apple Watch cannot be restored to previously-released OS versions once the public beta is installed.Apple
If you are still interested in beta testing watchOS 8, see this guide by 9to5mac on how to install the public beta.