Put your old iPhone and new iPhone close together
On your old iPhone, open the Apple Watch app
Tap My Watch, then 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
Keep your old iPhone nearby until the unpairing finishes and a backup is created
Turn on your new iPhone and sign in with the same Apple ID
Open the Apple Watch app on your new iPhone
Turn on Bluetooth and Wi-Fi on your new iPhone
Hold your Apple Watch near your new iPhone
Follow the on-screen setup prompts
Choose Restore from Backup if prompted
Select the most recent Apple Watch backup
Finish setup and wait for syncing to complete
If your watch was erased, pair it again by holding it near your new iPhone and following the prompts
If needed, erase the watch first by going to Settings on the watch, then General, then Reset, then Erase All Content and Settings