Use a tester to make sure the power to the outlet is indeed turned off (Image 1).
Remove the screws holding the cover plate and take the plate off (Image 2). Then test the outlet again -- and be sure to use the tester to probe all around the box to make sure there's no "juice" at all.
Remove the mounting screw and take the outlet from the wall.
Remove the wires from the outlet in the following order: Starting with the black (hot) wire, loosen the screw that holds it by turning it counterclockwise. Then do the same to remove the white (neutral) wire and lastly, the ground wire.
To attach the GFCI, start with the ground wire. Attach it to the green screw by forming a hook on the end of the wire, hooking it around the screw and tightening it, turning the screw clockwise.
GFCI stands for Ground Fault Circuit Interrupter, and it means an outlet that can prevent electrical shock in wet locations such as bathrooms, kitchens and outdoor areas.
A GFCI has two buttons: a test button and a reset button.
Note that the GFCI receptacle has two holes in the back; this is called a stab-in receptacle. Starting with the neutral wire, push the end of it into the hole on the side with the silver screw. Tighten the screw to hold the wire.
Repeat with the hot wire, attaching it through the hole on the side with the brass-colored screw. Tighten the screw.
Gently push the wires back into the box, then line up the mounting screw with the corresponding hole and tighten.
Replace the cover plate.
To test the unit (which should be done periodically), plug in an appliance such as a hair dryer and turn it on. While it's running, press the test button. The dryer should stop. Once you've ascertained that the GFCI is working correctly, unplug the dryer and press the reset button.
Remember: If you have any questions or fears about working with electricity, call a licensed electrician. This type of work can be very dangerous, and there's no use risking your life just to save a little money.