Depending on your definition, it's not really possible to improve the quality of the video in a basic sense, as you can only work with what data presently exists and any further lossy video compression will only further lessen the quality. 'Garbage in, garbage out' as they say. Now you CAN apply filters to the video to alleviate the effects of certain problems that exist in the video such as applying filters to lower noise/grain levels, macroblock lowering filters, light/dark/contrast/color filters, etc. But in doing so you are trading off some of the original accuracy and details of the original file. In some cases it's worth it though, for example in trading off some detail and further softening of the picture by filtering out heavy macroblocking.