Apple's latest iOS 9 has a new feature that will help burn a hole in your data plan. You'll need to know how to turn it off, otherwise you'll be spending a lot of money on data, or you'll reach your limit faster than you expect.

It's called "Wi-Fi Assist." The feature is on by default, and it will automatically switch you to 4G if it thinks the Wi-Fi you're connected to sucks and is too slow. To turn it off:

1. Go to settings.

2. Choose "Cellular"

3. Scroll all the way down to find "Wi-Fi Assist." Touch the button to turn it off.