Apps that refresh in the background with the latest data can churn up valuable cellular or Wi-Fi data, even when you're not using them. Some can be useful, while others may not be. For instance, while it may be useful for a mapping app to provide you with turn-by-turn directions on the road, you may not want your Stocks app to update if you don't need up-to-the-minute information on the financial market.

To change this setting, tap through to Settings > General > Background App Refresh, then deselect apps that you do not want to update in the background.