Not sure about that particular pump, but this is the way to convert most of them:

1. Unscrew the end of the chuck that attaches to your tire.
2. There will be a rubber gasket and a hard plastic part inside. Take them both out.
3. Flip both parts over and put them back where they were originally. For Presta you want the finger on the plastic part pointing in and the side of the rubber gromet that has the smaller hole pointing out.
4. Screw the cover back on.