Most mobile apps do not have a "Quit" button. Instead, use one of the buttons at the bottom of the screen: Back, Home or Recent.
These buttons may look different on your device, and they may be in a different order.
Tap the Back button until you are returned to your Home screen.
Tap the Home button, usually the middle button, to return immediately to your Home screen.
Tap the Recent button to see your recently used apps, and either tap the X at the top of the NextRadio window, or swipe the NextRadio window left or right to close it.
Note that this does not stop the radio! To stop playback, press the Stop button before exiting NextRadio.