If you also need to securely erase your Apple Watch, you can use these steps to unpair your devices.
Tap Settings > General > Transfer or Reset [Device]
Then tap Erase All Content and Settings.
If asked, enter your passcode or Apple ID password
Wait for your device to erase
Depending on your device, it can take a few minutes to completely erase your data.
If you can't erase your iPhone, iPad, or iPod touch
- Learn what to do if you don't remember your passcode for your iPhone, iPad, or iPod touch, or your device is disabled.
- If you forget your Screen Time passcode, you need to erase your device, then set it up as a new device to remove the Screen Time passcode. Restoring your device using a backup won't remove the Screen Time passcode.
If you still need help, contact Apple Support.