https://wccftech.com/can-i-downgrade-watchos-7-final-back-to-watchos-6-on-apple-watch/

Today, Apple has seen fit to release watchOS 7 to the public with a handful of new features. watchOS 7 on the Apple Watch extends the wearable's functionality both in terms of health and general utility. However, initial releases are crawling with bugs and over time Apple releases several updates to make it more stable. Users looking to downgrade to watchOS 6 from watchOS 7 should know that you cannot do that.

As mentioned earlier, you cannot downgrade watchOS 7 to watchOS 6. If you're not a big fan of the watchOS 7 and afraid if the new operating system is filled with bugs, you can always wait to install a more stable build in the future. However, as of now, there is no way that will allow you to downgrade to watchOS 6 from watchOS 7.