https://wccftech.com/downgrade-from-watchos-8-beta-to-watchos-7-is-not-possible-on-apple-watch/

If you are hoping to downgrade from watchOS 8 beta to watchOS 7.x on Apple Watch, please know that you cannot do that.

It is no secret that watchOS updates cannot be dealt with in the same manner as iOS and iPadOS updates. You cannot jump from one firmware to the other as you please. If Apple releases an update, you either have the option to stick around with the older firmware as long as you like or just install the currently available update, whatever it might be.