Change Apple ID on Apple Watch

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. See:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Here are two ways your ex-wife can unpair the watch without access to the paired iPhone:

• See the 'How to erase your Apple Watch if you don't have your iPhone' section of:

Unpair and erase your Apple Watch - Apple Support

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

you’ll have to factory reset the watch in order to pair it to your iPhone. if you want to do it from the watch, go into settings then general an scroll to bottom and hit reset.

once it’s reset you’ll be able to pair it to your phone and Apple ID.🙃