iPhone settings are displayed. Scroll down the page till you see iTunes & App Store.
Click iTunes & App Store.
iTunes & App Store settings are displayed. As you can see, the toggle for 'Use Cellular Data' is switched on. Slide this toggle to the left to turn it off as shown below.
The toggle for 'Use Cellular Data' is now turned off and automatic downloads for apps, music and books as well as app updates would not happen when your device connects to cellular data thus saving your data plan from running out.