For Windows:
Minimize all windows. (That means click the little "-"
symbol in the upper right hand corner
of any window in your screen.) Then
click anywhere on the blank part of the screen with the right side of your mouse...
...Select "Properties," from the
little menu you'll see and you'll find THIS box:


After you click "Apply," the screen will go blank for a few seconds...(DON'T
PANIC!!!)
...and then light up again with the selected settings. You will be
asked if this is
OK, and if it is, simply click "yes," and close out the
boxes by clicking the little
"x"s in their upper right hand corners.
Then to make the windows you've minimized appear again, look for little rectangular boxes at the bottom
of the screen and click on the little square boxes. Simple,
huh?
For MAC:
In OS X, click on the little green apple in the upper
left hand corner of your screen, then on
"System Preferences" You'll find
THIS box:

Once you click on "Displays," you'll find THIS box:
...this tutorial will be refined for clarity.