Turn the fan off with the wall switch, remote or pull chain. Wait for the blades to come to a complete stop.
Turn the rotation switch on the wire housing of the fan to it's opposite setting. This is a vertical slide switch with two positions. Generally the down position will cause the fan to blow downward and the up position will create an updraft. Do a visual check on the rotation. When the fan is rotating in the direction of the high side of the blade tilt, it blows downward. When the low side of the blades are leading, the fan sucks air upward.
Turn the fan back on. The fan blades will be rotating in the opposite direction. Set the fan to low speed with an updraft for use with heating.