If you forget your passcode on Apple Watch, or you’ve entered an incorrect one too many times, you can use the Apple Watch app on the iPhone to enter the passcode one more time. If you still can’t quite remember it, though, you’ll need to erase and restore the erased Apple Watch. This sets it back to factory default settings, and takes all the content off of it.
So how do you get all your stuff back onto the Apple Watch?
“Apple Watch backups are included as part of your iPhone backups to iCloud or iTunes,” says the Apple Watch user manual, “but you can’t view information in the backups in iCloud.”
When and if you un-pair your Apple Watch from your iPhone, a backup happens; you don’t even have to do a thing. To get your content back on it, simply launch your Apple Watch app on the iPhone and choose Restore from Backup. You’ll then have a chance to choose which backup you’d like to use. Do so, and your Apple Watch and iPhone will connect and put all your things back where they were when you backed it up. Hooray!
Source: Apple Watch user guide