Brightness is sometimes controlled by video effects in non-linear video editing software and sometimes in some type of built in functionality mastering images. In Windows Movie Maker it is found only under the video effects banner, which gives one option for increasing brightness and one for decreasing brightness. It is harder to use the brightness features in Windows Movie Maker because it does not let you alter the degree of it in a specific way. This makes the brightness factor impractical at times because its base amount is far removed from the original picture dynamic and cannot be used simply to fix some videographic problem. There is a way, however, to add extra brightness or decrease brightness in an excessive way beyond the base amount available in the video effects menu.
Go ahead and select clip in the Timeline that you wish to alter. Right click it and select Video Effects from the pop up menu. Here you will find the normal Video Effects window with the available clips on the left hand side and those on the selected clip on the right. Here you are going to find Brightness, Increase and Brightness, Decrease in the left hand column near the top and below Blur. Normally you would just decide which one you wanted to add to your clip, select it, and then hit Add in the center. Instead you are going to select either one of these brightness options and add it several times to the right hand column. Each time you add this brightness video effect you are going to either brighten or darken the image thoroughly. You can do this up to six stages, each one adding the full power of either brightness or darkness to the image that has already been altered. Try to just add each level of this one at a time and then check how the clip looks each time. This way you will be able to find the right balance that you want. Usually you will not want to use this much brightness or darkness unless you have a severely under or over lit image. This is especially true of over brightening, which brings a white fog all over the altered image.