how do i unpair a apple watch from a broken phone

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

I don't believe there's a way to unpair your Apple Watch without your iPhone, but you can erase your Apple Watch. This should do the same thing.

Read from the 'How to erase your Apple Watch if you don't have your iPhone' section in this article: Unpair and erase your Apple Watch - Apple Support

-- Jay (they/them)