Go to the main screen and click on one of the game buttons. If there is a game of that level in progress you'll be prompted to continue with it or start a new one.
The problem is that on the latest versions, there's no menu button to push to return to the main menu like there was on the earlier versions. So a person is stuck in the game mode without being able to get out of it until the game is over. When I've also quit the game and tried to reopen the app, it has just taken me back into the recent game that I've wanted to quit. So the real problem is that there is no "menu" button on the latest app version that would allow the user to easily go back to the main menu.
As I said in the other post: the iPhone version has a menu accessible via the "curled corner" on the lower right.
Also, depending on the version of iOS you are running, the way to actually quit the app may be different, but in most cases it starts off with double clicking the Home button and then closing it from the list of the running apps that comes up, and not just clicking it once to go to the home screen.