I need to pair my Apple 4 watch with a new iPhone. I no longer have my old phone.

1. On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. 

1. Type your password if prompted.
2. For GPS + Cellular models, choose to keep or remove your cellular 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 might need to contact your carrier to cancel your
  • cellular subscription. 

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

Update the Watch to 7.2 and then pair it.