Turn the electricity off to the furnace. Lift up the access door, and pull it out to remove it. The flame sensor and ignitor can be seen inside.
Pull out the plug-in connector from the socket by hand, and then remove the screws holding the ignitor to the metal bracket. Remove the ignitor.
Attach the screws of the new ignitor to the metal bracket, and push in the plastic plug-in connector into the socket. Replace the burner door, and turn the electricity back on.