Question: "My 10 slide presentation has fonts; Arial and Calibri. My header and body text are in Arial. The text boxes and shapes (including SmartArt) is in Calibri. I want to change all Calibri to Helvetica. I don't like Calibri. It's boring and too common. How do I do it at once?"
Solution: Under the Home Tab -> Replace (on your extreme right) -> Replace font (click down arrow to choose this option) -> Choose which font you want to replace with which new font -> Done! (MS PowerPoint 2010)
You can use this technique to change any font you have used in your presentation.