Well, technically, there's no such thing as over-exposed or under-exposed video. But you can certainly darken things that are too light or lighten things that are too dark.
That's, of course, to a certain extent. If the pixels in your video are washed out (completely white) because there was too much light or the pixels are so black that the there is no color information to work with, no program can work miracles.
Meantime, if you're looking for a good, basic guide for the program, you should check out one of my recent releases on Amazon. The are also lots of free, basic tutorials and tips on the products page at http://Muvipix.com.
Apart from Steve's advise take a look at the Shadow/highlight effect... it does a good job of bringing up the shadows and toning down the highlights.
Along with Shadow & Highlight, I also use Levels. Unfortunately, PE only offers Auto Levels (unless they added the manually controlled Levels in PE7). I find that Auto anything in Effects will be pretty useless, unless the lighting on a Clip is 100% constant. What happens with any of the Auto Effects is that the footage begins to "pulse," or "strobe." This is true for PE and for PrPro.
With "overexposed" video, it can sometimes be useful to place a copy of the Clip above it on, say Video Track 2. Apply Shadow & Highlight with different settings (experiment) and then slowly turn down the Opacity of the copy of the Clip. With PrPro and the manual Levels Effect, plus a few more, there is a lot that can be done to "salvage" footage that has blown out highlights. PE is a bit more limiting, because of the lack of manual Levels Effect.
For "underexposed" video, when it's brightened, video noise can often be detected. I find that an add-on product, Neat Video goes a long way to reducing this video noise. It is available as a plug-in for PrPro (probably will work in PE, but do not have it on my laptop with both programs), and/or a stand-alone video processor. The Rendering and/or Export time will go up with this Effect applied.
thank you for the help. I will follow up this advice.
thank you very much for the info. I will try this very soon.
thank you for your information. I hadn't thought of "Levels"
I will try this real soon.