Wiring for a ground fault circuit interrupter (GFCI) depends on where it falls in the circuit and whether you want it to protect the other outlets.
Turn off the power and look into the box to see whether the receptacle is at the start, at the end, or in the middle of a run. If it is in the middle or at the start, it can protect other outlets. Consider any drawbacks to protecting outlets in the circuit after the GFCI. Do you risk cutting off power to a freezer if the GFCI trips, for instance? The following shows how to protect a single outlet.
About 25 minutes to remove a receptacle and install a GFCI
Screwdriver, side cutters, strippers, lineman's pliers, voltage detector
Stripping and splicing wire
Lay a towel or small drop cloth on the surface below the receptacle.
GFCI receptacle, wire nuts, electrician's tape
If only one cable enters the box, the receptacle is at the end of the line. Connect the black wire to the BLACK LINE terminal and the white wire to the WHITE LINE terminal. Connect the ground wire to the ground screw, pigtailing it to the box if the box is metal.