iOS lets you specify which application can refresh in the background and use internet and which app is not allowed. To turn off background app refresh on iPhone 6, do as follows:
1. Open the Settings application on your iOS device (iPhone, iPad, or iPod Touch)
2. Go to the General settings
3. Scroll down to the Background App Refresh settings and open it
4. It you want to completely turn off background app refresh and don't let any app use internet (on Wi-Fi or Cellular) while running in the background, simply toggle off the Background App Refresh button.
5. Otherwise check the list of applications which consume data in the background, and toggle off the apps that are not allowed to refresh in the background.