factory reset someone else's iphone 6 when I do not have password or their apple ID

You don’t. That’s a theft deterrent mechanism, and only the person holding the Apple ID and password associated with that Activation Lock, or Apple, can remove it. Either the Apple ID holder releases it, or you produce legal or original purchase receipts for consideration by Apple, or you recycle the device.

Not possible. If your iPhone was missing or stolen, would you be OK with some other person you do not know resetting the device so they could use it?