https://wccftech.com/downgrade-from-watchos-9-beta-to-watchos-8-on-apple-watch/

Whether or not you can downgrade watchOS 9 beta to watchOS 8 on Apple Watch, we’ll give you a straight answer.

Rather than babbling about how the Apple Watch cannot be hooked up to a PC or Mac, we will just let you know that you cannot downgrade watchOS 9 beta back to watchOS 8 no matter what you tried or did at home. Once you are on watchOS 9 beta, you have to install every single beta update until the final version becomes available to you later this year.