Unlock an iPhone 5s

Ok so I hate ATT but I have an ATT 5s that i bought and I want to get it unlocked. ATT won't do that for me since I'm not the first owner. Anyone know how to unlock an iPhone, preferably for free, or how to get ATT to unlock it for me? I had an ATT account way back in the day so technically I'm a previous ATT customer but the ATT rep didn't like me bringing that up I must have sounded like a smart ass lol

Any ideas or recommendations would be great!! Thanks