Note: If the existing electrical box is too small to accommodate a GFCI outlet, or if the house wiring is aluminum instead of copper, you may need to hire a qualified electrician to do this job.
Turn off the power at the circuit-breaker box. If the circuit breakers aren't labeled, you can locate the proper switch by plugging a radio into the outlet you plan to change. Turn off the switches until the radio goes off. Then place a piece of tape over the switch to make sure no one accidentally turns it back on while you're working on the outlet.
Remove the outlet cover plate and the screws holding the outlet in place.
Test the outlet with the circuit tester to be sure the power is off.
Disconnect the wires from the outlet.
Separate the wires from the box into two pairs. One set of wires will be the "line," or power supply. The other set will be the "load," which carries power to additional outlets on the same circuit. A GFCI outlet, properly installed, will protect all the outlets on the "load" side.
Connect the power-supply wires to the terminals marked "line" and the load wires to the terminals marked "load." Connect the white wires to the silver screws and the black wires to the brass or gold screws. The outlet may also indicate appropriate color connections.
Connect the bare ground wire to the green screw.
Put the outlet back into the box, screw it into place and attach the cover plate.
Turn the power back on at the circuit-breaker box, and plug a radio into the outlet.
Press the black button on the outlet. If the radio turns off, the outlet is working.
Press the red button on the outlet to reset it. The radio should come back on.