Switching between Metro UI and the desktop is very easy, and you have a couple of options.
Use the Windows key
If your device has a hardware keyboard with a Windows key, pressing the logo key toggles between Metro UI and the desktop mode.
Use the Desktop Tile and Start Menu
When you are on Metro UI, touch or click the desktop tile to switch to your desktop environment. When you are on the desktop, move the mouse cursor to the lower left corner until the Start icon pops up, click on the icon to switch to the Metro UI. If you have a touch enabled device, you can use touch gesture to accomplish the same task.
Use the Start button from the Charms Menu
Move your mouse cursor or swipe to the right side of the screen until the Charms menu appears. Click on the Start button (window with four panels) to switch to the Metro UI. Use one of the options mentioned above to switch to desktop.
Windows 8 Hotkeys
In an article “Windows 8: 20 Tips and Tricks” at GIZMODO, David Murphy lists several Windows 8 Hotkeys you may find very useful.
You can download a keyboard shortcuts pdf file from Kent Walter’s post, “Getting around in Windows 8”.
To see my other Win8 HowTo blog posts, click here.
If you have a different or better way, share it with us.