watch battery drains fast after update to watchOS 10

This is actually normal. It takes Apple a little while to fix battery life issues after software updates. It should get better over the next few months as Apple releases updates. You can turn on low power mode on the watch to help battery drain less. Unfortunately, Apple does not allow downgrading watchOS, so you’ll have to wait. You can also see about getting your watch’s battery replaced because it could be having issues.