How to unpair Apple Watch from a lost phone?

Follow the steps here:

Unpair and erase your Apple Watch – Apple Support

How to erase your Apple Watch if you don't have your iPhone

1. On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. 
2. 
3. 
   
4. Type your password if prompted.
5. For GPS + Cellular models, choose to keep or remove your mobile data plan.

• • If you want to pair your Apple Watch and iPhone again, keep your plan. 
  • If you don't want to pair your Apple Watch and iPhone again, remove your plan. If you aren't going to pair with a different watch or iPhone, you may need to contact your network provider to cancel your mobile data subscription. 

1. Tap Erase All to confirm. This resets your Apple Watch to the factory settings.

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.