Screen time

You will need to restore the device on iTunes to factory settings. You will not be able to restore a backup as this will also restore the screen time passcode.

https://support.apple.com/en-ie/HT204306

Make sure to choose a passcode that's different from the passcode you use to unlock your device. If you can't remember a restrictions passcode that you set on a previous version of iOS, or you forgot your Screen Time passcode, you need to erase your device, then set it up as a new device to remove the passcode. Restoring your device from a backup won't remove your passcode.