How can I unpair my Apple Watch if I no longer have the iPhone it was paired with?

See this article and scroll down a bit, and follow the instructions

Unpair and erase your Apple Watch - Apple Support

Or;

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.