If you forget your new iPhone passcode, you can reset it with your old one within three days.To unlock your iPhone with an old passcode, tap 'Forgot Passcode?' after multiple incorrect attempts.You can create a new passcode using the old one, which can be used for up to 72 hours.To manually expire the old passcode, go to Settings > Face ID & Passcode and select 'Expire Previous Passcode Now.'