You can't improve the quality of a webcam.
But there are a few general tips:
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Is the video dim? - There isn't enough light in your room. Try turning on more lights, or move to an area where there is more light.
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Is your image dim? - This could be caused by having a bright light source behind you. The camera adjusts its sensitivity to the background instead of you. Try positioning yourself so the light source is in front of you.
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Is your video grainy? - While this could be caused by a low quality camera, it can also be caused (or made worse) by not having enough light. To compensate for the lack of light, cameras often adjust their sensitivity which creates more grain or "noise" in the image. Try turning another light on, or...