How to correct over-exposure with Gimp?

About all I know about photo editing is cropping and adjusting brightness and contrast to the standards of an amature. However, I would like to try something a little more complex. Can anyone help me?

I have a HP Scanjet 3970 scanner, using Sane scanner software, and I want to scan some old slides taken with and old kodak 3-D camera. The slides are very crisp and clear. But when I scan them they appear over-exposed.

I don't know if I should configure Sane to scan the slide differently or if I should use Gimp to correct the image after it is scanned.

How can I correct this? I have messed around for hours and can not make my images look better. I suspect that I need to use the histogram but I also suspect that the histogram is beyond my abilities.

As long as detail isn't being lost in the highlights the post correction will work. The tool I would be inclined to use is colours->Levels...

Otherwise and probably better anyways is to set the scanner up properly - which sane frontend are you using? some have more control than others.