How to force quit an app in iOS 7
The process for force quitting an app has changed slightly in iOS 7. Follow these steps to close an app which is misbehaving, and return it to the default state.
1. Enter the app switching screen by double tapping the home button. On an iPad, you can also swipe up on the screen with four fingers (Multitasking Gestures must be enabled in Settings for this).
2. Flick up on the image of the app you want to quit. Make sure you're flicking up on the screenshot of the app itself, not the icon at the bottom of the screen.
You can force quit two or even three apps at a time using this method, though I can't actually think of a situation in which you would need to do that.