Q: how to unlock iphone 5?

only the carrier to which it is locked can unlock it and it is likely the original owner will need to mae the unlocking request.  If you are talking about the activation lock then read http://support.apple.com/kb/ts4515

Just turn it in to the police.

Nobody here would know it, and Apple will not provide it.  You have a device that you cannot use.

It doesn't belong to you. That's why you can't use it. Turn it over to the police.