Remove the Cover and Separate the Wires
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.