Apple created app store promo codes in order to offer a way to distribute application for reviewing purposes. To use promotion codes can somehow allow reviewers to get the app on their iPhone, without having to pay for it. And unluckily, it can only be use in US area. There are many articles talking about how to redeem the app on iTunes, here we will tell you how to do it on iPhone.
First, open your App store on you iPhone, normally you can find the icon on the first page.
Next, go down to the bottom of App Store you can find the button of Redeem:
Click it, and you will go into redeem view:
In redeem view, input your promotion code in the code text box and click go:
After loading, a window will pop up, select continue to go next steps:
System may ask for your Apple ID password, input your password and click ok:
Then WAIT~~~ be careful it may take minutes to wait, seems nothing happen, but please wait till the system happen to loading:
At last you will reach this step the app your wish to redeem will be downloading: