How to change my itunes back to us?

Ok so i'm on my phone and i wanna buy an app but every time i click buy it says i need to change it back to US and i don't know how to change my itunes back to US on my phone. If it matters i have the iphone 5. I tryed plugging in my phone into my computer and changing it but it only changes on my computer not on my phone.

Open up the itunes store then scroll down to the bottom of the page. On the right side it says "manage" and underneath that it says "Change Country". Click it and change it.