Remove the outer-door of the furnace to access the inside. Most models of furnace have a door the simply slides off.
Look inside for the metallic box known as the high-pressure limit switch. Generally it's located in the upper reaches of the area behind the door.
Slide back the cover to the switch. Expose the dial and actual switch itself.
Reset the dial settings by moving the metal arms. Place them (from left to right) on 90, 130 and 200 degrees Fahrenheit respectively. The first two control the fan, while the third shuts off the burner when it gets too hot.
Pull the switch out if it isn't already. Leaving it in will force the fan to run automatically regardless of furnace temperature, blowing cool air through the house.