Open the video footage in your preferred video editing program.
Drag the video clip from the "Import" or "Video Clip" window to the program's "Editing Timeline." This timeline, which is comprised of a number of video and audio tracks, progress and time duration bars and editing tools, is where the actual video editing is done in the program.
Look for the "Brightness" feature in your video-editing program. If you are using the latest version of the native Windows program "Windows Movie Maker," this feature can be accessed by clicking the "Visual Effects" tab on top of the screen, then selecting the "Brightness" button, which has a sliding bar that allows increasing or decreasing the selected video's brightness. If you are using Mac's iMovie, this feature can be accessed by clicking the program's "Brightness" slider. Alternatively, you can use the standard effect called "Hard Light" under iMovie's "Video Effects" menu to help improve the poor lighting of your video. Use your program's "Preview Window" to view how your video looks with the applied effect.
Render the edited video to finalize the brightness effect applied to it, then export the video to your preferred video file format, which typically include MOV, WMV, AVI or MP4.