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
Then confirm that you want to erase your device. Forgot 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.