watchOS 10
Pair Apple Watch Ultra with a new iPhone
If your Apple Watch Ultra is paired with your old iPhone and you now want to pair it with your new iPhone, follow these steps:
Use iCloud Backup to back up the iPhone currently paired with your Apple Watch Ultra (see the iPhone User Guide for more information).
Set up your new iPhone. On the Apps & Data screen, choose to restore from an iCloud backup, then select the latest backup.
Continue iPhone setup and, when prompted, choose to use your Apple Watch Ultra with your new iPhone.
When iPhone setup completes, your Apple Watch Ultra prompts you to pair it with the new iPhone. Tap OK on your Apple Watch, then enter its passcode.
For more information, see the Apple Support article How to pair your Apple Watch with a new iPhone.