Update the new iPhone to the latest iOS version
Turn on Bluetooth and Wi-Fi on the new iPhone
Sign in to the new iPhone with the same Apple ID used on the Apple Watch
Keep the old iPhone nearby if it is still available
On the old iPhone, open the Watch app
Tap All Watches
Tap the info button next to the watch
Tap Unpair Apple Watch
Enter the Apple ID password if prompted
Wait for the Apple Watch backup to finish
Turn on the Apple Watch and place it near the new iPhone
On the new iPhone, open the Watch app
Tap Start Pairing
Follow the on-screen pairing steps
Choose Restore from Backup if prompted
Select the most recent backup
Enter the Apple Watch passcode if requested
Complete setup and adjust settings as needed