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Settings -> Cellular will also show you what apps are using the most data so you can choose to disable them or not. If you disable they will only pull data while on wifi so beware if you have a wifi cap as well. The amount shown is cumulative so you should reset it if you want to monitor at certain times. We reset each month on the account date so we can keep track of data usage.Go to Settings > Cellular. From there you can toggle off apps that you don't want using cellular data. While you're there I recommend scrolling all of the way down and toggling off Wi-Fi Assist.