For devices that have a Home Button: 

1.  Press your Home button twice quickly.

2. All your open apps will be displayed. Swipe left or right to locate your app.

3. Swipe up the app's preview image.

4. Return to the Home screen and re-start the Game app.

For devices that don’t have a Home Button:

1. From the Home screen, swipe up from the bottom of the screen and pause slightly in the middle of the screen.

2. Swipe right or left to find the app that you want to close.

3. Swipe up on the app's preview to close the app.