If you've exceeded your data limit since updating to iOS 10 on your iPhone (and it's out of the norm), here's why:
Turn off wi-fi assist
If you don't want your iOS device to stay connected to the Internet when you have a poor Wi-Fi connection, you can disable Wi-Fi Assist. Go to Settings > Cellular. Then scroll down and tap Wi-Fi Assist.
Restrict cellular data usage
There are some apps, like Netflix, you should really never use over a cellular connection. You can easily manage which apps are able to use data. Go to Settings > Cellular and scroll down to the list of apps. Turn off anything you don't regularly use, as well as any especially data-hungry apps like those that stream music and video.
Turn off background refresh
Many apps also use background refresh, which allows them to use data even when you aren't using them and can eat into your monthly allotment. To turn it off, open your Settings and scroll down to the list of apps. Select the ones you want to disable background refresh on and the option appears, along with the option to turn off cellular data.
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