Most laptops have the ability to view both laptop and TV/projector (or other VGA, HDMI etc. device) to "mirror" each other (same image to TV as on laptop screen), to "extend" the displays (giving one long desktop across the two screens) or to have one or the other on at any one time (one will turn off and the other will be the "primary" screen - often used if you want your laptop screen off to watch a film on a big TV, for example).
Most laptops you have to hold Fn + one of the F keys - F5 is quite frequently used. You should get an overlay pop up to show what is going to be displayed. The F key to look for will have what looks like two monitors next to each other or "LCD/CRT".
A number of graphics card manufacturers have written system tray applications where you can fully manage the output to an external display in a couple of clicks. These are usually installed during the driver install process.
In windows 7 pressing the windows key + P brings up an alt-tab style display switcher. Dunno if this works for windows 8 but try it. Also some laptops have a hot key, in particular look for symbols that look like monitors on your F? keys. Some graphics drivers have support for assigning global key combos to display configurations as well, check your driver control panel. – Jason C – 2014-06-29T00:22:53.370
Do you mean it's hooked up, but you're only displaying on a single screen at any given time, and want to change which screen? – Collin Grady – 2014-06-29T04:10:47.597