For Apple (iOS) devices:
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 Android devices
1. Go to "Settings" > "Applications" > "Manage Applications"
2. Select the app and tap "Force-Stop"
3. Return to the Home screen and re-start the game app.