Couple ways: you can export the slides as jpeg stills from powerpoint into a folder, take the folder into photoshop, apply an action to invert all the images, or flip them by hand. Save the folder. Then use the inverted images in a new powerpoint. Works, and costs nothing extra, but takes some time, and limits how flexible you can be about changes.
Method I now use: a scan converter with the ability to do the reversal inside itself, put between the laptop and the prompter. Specifically, a Hall Research model SC-VGA-2B.
There was one other way to do the inversion within Powerpoint itself, but I’m drawing a blank on that right now. Maybe Microsoft Producer has that functionality in it, that’s a free download from Microsoft.