Unlike film photography, the beautiful thing about digital photography is that everything's there, even if it's underexposed. I hope you have the original untouched images.
Download the GIMP from
http://gimp.sourceforge.net
A windows version is available.
Now I am not an expert, but I will tell you what i know. I just downloaded an underexposed image from the internet for this example.
Open the image in the GIMP
Click on Colors->Levels
Levels dialog box will open. Observe the graph and the three arrows (like fulcrums) below it, these are for Shodows, Midtones and Highlights
Move the dialog box out of the way
Drag the arrows on the right (highlighted in the snapshot below) towards the curvy graph, starting with the centre one and then the one at the end. After you move the one at the end towards the graph you will have to readjust the centre arrow.
Once satisfied click OK, and then File -> Save A Copy