How to factory reset iPhone 6 without iTunes/computer/passcode?

So, found a phone on the street today beat the hell up and I was thinking I would just use it for spare parts. When I got home I took off the case and realized that the case was the only thing broken (Otterboxes, lemme tell ya) and the phone wasn't touched. I don't have a computer, pass, or iTunes.

You can't reset an iphone without the passcode/password.

That's the basic anti-theft mechanism, the whole idea is to make them worthless of stolen & that would be pointless if it was something that could be bypassed.

Note that you are committing a criminal offence by keeping the phone - "Theft by finding" is still theft.
Hand it in to an Apple store or the police.