Pair new iPhone to Apple Watch

You will first need to erase your old Apple Watch, in order to be able to pair it again. See this instructions on how to do that

f you don't have your old iPhone or you erased it

Your Apple Watch might still be paired with your old iPhone. Use the steps below to unpair:

1. Erase your Apple Watch. 
2. Set up your new iPhone and sign in to iCloud. If you've already set up and signed in, skip this step.
3. Open the Apple Watch app on your new iPhone, then pair your watch with your new iPhone.
4. Try to restore from a backup. Make sure that your iPhone and Apple Watch are up to date; if they're not, you might not see your backup in the list. You will lose Activity, Workouts, or settings on your watch that changed between the last backup and now. If you use iCloud in iOS 11 and later, Health and Activity data is stored in iCloud and automatically kept up to date on devices where you're signed in with the same Apple ID. 

• • If you’re asked to restore from a watch backup, select the most recent backup. 
  • If you don't have a backup, you need to set up your Apple Watch as new. 

1. Follow the onscreen steps to finish setting up.
2. Start using your Apple Watch with your new iPhone. You might need to wait a few days for all your iCloud data to appear.

iCloud doesn't store your Activity Sharing data.

If you have your old iPhone and you didn't erase it

Unpair your Apple Watch from your old iPhone. Then choose one of these options depending on when you set up your new iPhone:

• If you just restored your new iPhone from a recent backup, pair your devices. Make sure that your iPhone and Apple Watch are up to date, otherwise you might not see your backup in the list. When you're asked, restore from a watch backup. Your recent Activity, Workouts, or settings content might be lost depending on how old the backup is.
• If your iPhone backup is several days or weeks old and you want to preserve as much content as possible, erase your new iPhone. Then transfer the content from your old iPhone to your new iPhone. You might lose some of the content on your new iPhone. Remember to pair your Apple Watch and iPhone again. 

How to pair your Apple Watch with a new iPhone - Apple Support