Force Quit apps on iPhone or iPad: How to close all apps on iPhone 6? You don’t need to Force Quit apps on iPhone or force your apps to shut down,but doing it sometimes helps. 
How to force quit an app on iPhone or iPad
STEP #1: Double click the Home button (or 3D Touch press the left side of the screen on iPhone 6s or later) to bring up the fast app switcher.
STEP #2: Navigate to the app screen you want to quit.
STEP #3: Swipe up on the app card you wish to shut down by flicking it up and off the screen.
Force quit three apps at once on iPhone or iPad: iPhone close all apps at once (Maximum 3)
How to close all apps on iPhone 6 (Maximum3): Force Quit apps on iPhone
STEP #1: Double click the Home button (or 3D Touch press the left side of the screen on iPhone 6s or later) open the fast app switcher on your iPhone 6, iPhone 7 Plus or iPad.
STEP #2: Now scroll to the iPhone or iPad apps you want to force quit
STEP #3: Now you have to use your three fingers simultaneously, swipe on the app icons you want to close or quit by flicking them up and off the screen.
How to force quit an app without using the fast app switcher
Force Quit apps on iPhone By holding down the Power and Home buttons.
STEP #1: Visit the app that you want to forcibly shut down.
STEP #2: Hold down the On/Off button until the “slide to power off” screen appears.
STEP #3: Hold down the Home button until you’re dropped back onto the Home screen.