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On the bottom section of the phone there is a button with a square on it. Just click it and you should be taken out of the app.
The first way closes the app and the second way totally shuts down the app from running in the background for the short time it does... About 10 mins and then it goes dormant anyway.If you are looking to close an app then you need to double tap the home screen- you will see your open apps- swipe to the one you want to close and then swipe the app up and that closes it