First of all I would like to give you a friendly warning. You see, unlocking an iphone is a little risky as it could harm your iphone if you dont do it correctly. Some people had to buy a new iphone after they tried to unlock their iphones themselves and that's because they ruined their iphones in the process.

Now, to unlock your iphone you could do it by messing with the Iphone's hardware (which I wouldn't recommend ,unless you are an engineer) or you could do it by using an iphone unlocking software.
The problem is that it is very difficult to find a good unlocking software for your iphone and some of the free softwares that are available online can be outright dangerous to your iphone. Most of the times, these free unlocking softwares either contain some sort of virus, which can render your iphone useless, or some tracking tool which spies on your usage.

The best thing would be to get an unlocking software from a reputable site such as 1Click Iphone Unlock . There you just download a software and then just follow the simple instructions to unlock your iphone.