Replacing a GFCI outlet.
Remove the cover plate, usually held on by 2 screws, although some just snap in place.
Before removing the outlet from the wall, be sure the power is turned off to the circuit you are working on. use a tester to make certain the power is off
Once the power is off, remove the 2 screws holding the outlet in place. Pull the outlet from the wall box. Retest that the power is off to all wires in the box.
It is important to note and mark which wires go where on the existing outlet. A GFCI outlet will have 2 sets of terminals. The first set will be marked Line, this is power in and out of the outlet. The second set of terminals will be marked Load, any wires attached to these terminals will be protected by the GFCI circuit.
Check out this video on changing out a GFCI outlet.