Spending money on a large data package for your iPhone 6 is not always the best option. If you use the phone mainly with a Wi-Fi connection, you can use a smaller data package–if you remember to turn off the phone’s cellular data function when you’re connected to Wi-Fi. Most carriers will not tell you that if you lose a Wi-Fi connection, even momentarily, the iPhone will revert to using your data without warning. Turn off the cellular data to keep the iPhone from going back to it when you lose connectivity.
Select the "Settings" icon from your home screen.
Select the "General" tab.
Select the "Network" tab.
Touch the "OFF" button in blue next to the box labeled "Cellular Data."