how to reset a restriction code on iphone 6splus. the person that i bought it from has forgotten it as well so now i'm lost

Hello,

If you forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode. Restoring your device using a backup won't remove the Restrictions passcode.

From:

Use parental controls on your iPhone, iPad, and iPod touch - Apple Support

If you are seeing the need for a Restrictions passcode on a phone that you purchased from someone else, that is going to be the least of your worries. I'll be willing to guess the phone is also Activation Locked, and at a point you will need the former user's Apple ID and password. Make sure the seller performs all of the steps here before you take possession of the phone. What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support