How to reset the 4 digit restriction code on iPhone 6
Lost my 4 digit pass code to unlock the restrictions on the iPhone 6. How do I reset this 4 digit code?
iPhone 6
You cannot reset it. You'll need to restore the iPhone to factory condition to remove the Restrictions passcode.
If you have a backup made before the restrictions passcode was put in place, you can use it to restore the iPhone.
Otherwise, any backup made after the restrictions were in place will put the passcode back.
From Use parental controls on your iPhone, iPad, and iPod touch - Apple Support
If you lose or forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode.