https://wccftech.com/downgrade-watchos-6-not-possible/

In case you are wondering, you can’t downgrade watchOS 6 to watchOS 5 on Apple Watch, and here’s why you can’t.

If you are a brand new Apple Watch user then you must be super psyched about watchOS 6. The new features are definitely something that will make anyone drool. But, if you don’t find the update in good taste, or you just learned that the update made your Apple Watch slightly slower than before then the only course of action you can take right now is downgrade watchOS 6 to watchOS 5.