On the new phone, install and open the Apple Watch app
On the new phone, sign in with the same Apple ID used with the Apple Watch
Turn on Bluetooth on the new phone
Keep the Apple Watch and new phone close together
On the Apple Watch, confirm it’s on and unlocked
If the Apple Watch is paired to the old iPhone, open the Watch app on the old iPhone
In the Watch app, go to My Watch and select the watch
Tap the info button (i) and choose Unpair Apple Watch
Choose to keep the cellular plan (if applicable) and follow prompts
On the Apple Watch, start pairing again when prompted (you should see the pairing screen)
On the new phone, tap Start Pairing in the Apple Watch app
Hold the new phone over the animation on the Apple Watch to pair
Follow the prompts to set up the watch (restore from backup if offered)
If prompted, enter the Apple ID password and complete any carrier/cellular setup
Wait for synchronization to finish before using the watch