Update your old iPhone and new iPhone to the latest iOS version
Keep your Apple Watch and old iPhone close together
On your old iPhone, open the Watch app
Tap All Watches
Tap the info button next to your watch
Tap Unpair Apple Watch
If prompted, enter your Apple ID password to remove Activation Lock
Wait for the watch to finish unpairing and create a backup
Turn on your new iPhone and sign in with your Apple ID
Open the Watch app on your new iPhone
Keep your Apple Watch near the new iPhone
Tap Start Pairing
Follow the on-screen instructions
Choose Restore from Backup when prompted
Select the most recent backup
Enter your Apple ID password if asked
Wait for the pairing and restore process to complete
Keep both devices near each other until setup finishes
If your watch does not pair, erase the watch and try again
If needed, reset the Apple Watch and pair it as a new watch