I found an iPhone X that’s not activated. It has a email already in it. How do I change the email

You won't be able to remove the activation lock unless you can contact the rightful owner and they are willing to help you (perhaps if you pay them for the phone). Note that the iPhone is not yours. It undoubted was stolen and, then, used to scam an unsuspecting buyer into buying a useless phone. Since the phone is not yours, take it the appropriate authorities. Maybe they can find a way to return it to the rightful owner. Note that you almost certainly are in possession of stolen property.

You can't. Turn the phone over to the local authorities.