Here are four ways you can try closing down Metro apps in Windows 8. 1. Bring up the Open Apps sidebar on the left side of the screen, then right click on an open app to shut it down. 2. Put your mouse over the top of your screen, then click and drag to the bottom of the screen. 3. Use the App Control bar to shut down Metro apps. 4. Use this keyboard combination: ALT+F4.
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Windows 8 Tips & Tricks | Tom's Guide