How to unlock an iPhone so that I can use any SIM

How to unlock an iPhone so that i can use any SIM in my I phone I want to Unlock my iPhone from O2, Three, EE, Vodafone & more. How to unlock an iPhone 4, 4S, 5, 5S, 5C, 6 or 6 Plus from any carrier it's locked to, including O2, EE, Vodafone and more. Thanks