My apple is connected to my old phone and old Apple ID how do I connect it to my new iPhone and Apple ID

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;

Set up and pair your Apple Watch with iPhone - Apple Support