How to take a phone number off a iphone 5?

I found a lost iphone and I couldn't find the owner, now how do i take they are number off and put my number in it?

Added (1). Not a contact the actual number connected to the phone

Go to the contact. Click edit then go all the way to the bottom and touch delete contact

You would have to bring your stolen phone physical store of the carrier as they would have to re-program the phone