How to reset the screen lock pin on an iPhone 6+ that is not paired with your iCloud account?

If you buy a used iPhone, take it home, then find out that the screen is still locked with the previous owner's PIN, how do you get into it?

I can't ask him for the PIN directly because a) I don't know him, and b) I'm told that he's abroad with no way to be contacted (I purchased it from his brother).

Note: the "Medical ID" page shows the name and weight of the person who owned the phone *before* the brother of the guy who sold me it, so I don't know if this helps any? (.so it was original owner > 2nd owner > 2nd owners brother > me)

Assuming that the phone isn't connected to any iCloud account, put the phone in DFU mode and restore in iTunes. If you find out that it asks for an iCloud account, you got scammed.