Back up your old iPhone
Update your old iPhone and Apple Watch to the latest software
Keep your Apple Watch and old iPhone close together
Open the Watch app on your old iPhone
Tap All Watches
Tap the info button next to your watch
Tap Unpair Apple Watch
Enter your Apple ID password if asked
Set up your new iPhone
Sign in with the same Apple ID
Open the Watch app on your new iPhone
Turn on Bluetooth and Wi-Fi
Keep your Apple Watch close to your new iPhone
Tap Start Pairing
Follow the on-screen steps
Choose Restore from Backup if prompted
Enter your Apple ID password if asked
Wait for syncing to finish