Try one of the following - in each case, you may wish to make an encrypted backup of your iPhone first:
- https://www.apple.com/support/iphone/backup/
1. Erase and reset both your iPhone and your Apple Watch and set both up as new devices:
Unpair your watch via the Watch app:
- Unpair your Apple Watch and iPhone - Apple Support
Restore your iPhone to factory settings and then set it up (without restoring it from backup):
- Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support
- Get started with iPhone, iPad, and iPod touch - Apple Support
Pair your Apple Watch with your iPhone:
- Set up your Apple Watch - Apple Support
2. Alternatively, erase and reset both devices, then partially restore your iPhone (excluding Health data and other sensitive data such as HomeKit data, saved passwords, etc) before pairing your watch:
Start by making an unencrypted backup of your iPhone:
- This must be done via iTunes on a computer (because iCloud backups are encrypted as standard);
- Do not select the option to Encrypt iPhone backup;
- How to back up your devices using iCloud or iTunes - Apple Support
Then restore your iPhone as new:
- Connect your iPhone to your computer and open iTunes;
- With your iPhone connected, force restart it: press and hold both the Sleep/Wake and Home button for at least 10 seconds, and don't release when you see the Apple logo. Keep holding the buttons down until you see the recovery mode screen;
- When you see the option to Restore or Update, choose Restore;
- After the Restore completes, set up your device - during setup, restore your iPhone from the unencrypted backup that you made earlier;
- If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support
Then pair your Apple Watch with your iPhone, choosing to set it up as a new watch (do not restore it from backup if offered the option):
- Set up your Apple Watch - Apple Support