I don't have the iPhone to which my apple watch is paired.

read

How to pair your Apple Watch with a new iPhone - Apple Support

focus on the section

"If you have your old iPhone and you didn't erase it"

On a computer, go to iCloud.com and sign in with your Apple ID.

Go to Find Iphone.

Select All Devices, then click your Apple Watch.

Click Erase Apple Watch.

Select Next until the device is erased.

Click the remove button next to your Apple Watch.