Pause/stop the app. Then just swipe your finger from left to right across the screen repeatedly until you reach the home screen. You can't really "close" apps. You just stop them doing whatever it is they do. You can also go into settings and set the clock to display first on "waking". Settings->General-> Date&Time->Time On Wake =ON Once this feature is on you just press the "sleep" button once to "sleep" the nano then again to "wake' it and the clock should be displayed.