How do I Network unlock iPhone 5s (allowing me to change carriers )?

I was gifted a iPhone 5s after losing my old phone the person didn't steal from anyone or anything it's just there old phone that they stopped using but I have metro pcs and the phone is locked by at&t and I would very much like any information regarding how to unlock it and allowing it to be used by any character

Go here and follow the instructions: https://www.att.com/...eunlock/#/
Though, just be advised that unlocking the phone doesn't guarantee that it will work with Metro.