In a full screen application, slide your mouse to the top of the screen to show thew menu bar. Tnen, slide full right to a blue box with 2 arrrow heads. Click the blue box to exit full screen mode. You can also slide to the application's "View" menu and select the choice to"Exit full screen".
Thank you for posting this answer. The issue I had was that I have a second display set ABOVE my laptop screen. So when I move the mouse up it just goes to the second display and nothing pops up. I had to turn on mirroring to get iTunes out of fullscreen mode.