How do I unpair my Apple Watch from an old (lost) I phone so that I can pair with my new phone?

To unpair and erase watch and remove Activation Lock.

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.

Then go here;

Pair the watch with my new phone.

There’s a whole section in the article linked above entitled: “How to erase your Apple Watch if you don’t have your iPhone”.