Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version)
Erase Apple Watch – Apple Support (UK)
Erasing all content and settings this way erases all media, data, and settings from your Apple Watch, but it does not remove Activation Lock if it has one. (no problem as she will use the same Apple ID)
Or you can erase and remove it from her account in Find My or https://icloud.com/find .
Wait for the process to finish, then set up your Apple Watch again. Set up your Apple Watch – Apple Support (UK)
When asked, decide to set up as NEW..
Then process the update
Remember the Watch must again be on its charger with the phone next to it