To close the app using a PC, all you need is to move the cursor to the upper right corner of the screen and click the X icon that appears there.

If you’re using a touchscreen device, just make a swipe from the top of the screen – the button to close the application should appear at the top of the screen.
You can also swipe from left to right: all the open applications will be displayed in the task window and you’ll be able to close the game.