See the illustration at:
http://www.axiomgear.com/manuals/enf...r-pro-road.pdf

Inside the head of the pump are two small pieces that need to be reversed to switch the pump to Presta. Unscrew the cap from the head and remove the rubber washer and plastic adapter pieces. Then flip both around and reinstall them. The plastic adapter should go in first and have the little nub pointing into the pump and the rubber washer should have the smaller hole pointing out toward the cap when configured for Presta valves.