Push the Wires Back Into the Box, and Replace the Cover
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.