How do I set up my Apple Watch with my iPhone

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.

Is the Watch new?

Start here => Get started - Apple Support

Is the Watch second hand?

Make sure it is unpair from the previous iPhone.