Keep your Apple Watch and new iPhone close together
On the new iPhone, sign in with the same Apple ID used on the Apple Watch
Turn on Bluetooth and Wi-Fi on the new iPhone
Open the Watch app on the new iPhone
Tap Start Pairing
If prompted, choose to restore from a backup
If the watch is still paired to the old iPhone, unpair it from the old iPhone first
If you no longer have the old iPhone, erase the Apple Watch on the watch itself
On the Apple Watch, open Settings
Tap General
Tap Reset
Tap Erase All Content and Settings
After the watch restarts, pair it again with the new iPhone
Hold the new iPhone over the watch animation
Follow the on-screen setup steps
Choose your settings and restore your data if available
Finish setup in the Watch app