Turn off applications on an iPhone by double-pressing the home button at the bottom of your screen. Swipe left through a series of thumbnail images to view open apps. Select and swipe up on your app’s image using two fingers to flick it off the screen.
There is usually no need to switch off an app unless it becomes unresponsive, according to Apple's support site. iOS is designed to support easy navigation and multitasking via app switching. In other words, apps do not automatically close when not in use; rather they run in standby mode in the background of iOS.
In the thumbnail view, the user can close any or all open applications. Changing the Background App Refresh settings can prevent an app from using too much data or battery power while it is in standby mode, says Nick Mediati at MacWorld. Background App Refresh can be turned on or off for specific apps. Occasionally some apps glitch, freezing or consuming battery power even when not in active use. When using the "force quit" method described above, the app is simply taken out of standby mode and reset. This causes any unsaved information gathered by the app in its current session to be lost.