What exactly are you trying to accomplish by unlocking this old phone? Are you trying to give the phone to someone so they can use it on a different network (other than AT&T), or something else? You can actually swap in a new SIM card (Google "how to replace the SIM card in an old iPhone") from a new provider, but there's a good chance you'll lose all of the contacts info from the old phone when you do this. That said, I wouldn't use an iPhone 5 nowadays due to the security & compatibility issues raised in the posts above. You're better off just buying a new iPhone 8 (the 6's and 7's are going to be sunsetted this fall as well). That's the least-expensive current model that allows for all of the necessary updates.