Use the curves modifier
The curves modifier can be found in the video sequence editor. Inverting the pixels is easy by changing the shape of the curve(s). The left point (representing the darkest pixels) originally at 0 should be set to 1 (to map those pixels to be the brightest) and vice versa the right curve point from 1 to 0. Here is a gif:
This gif only shows the work for the C-channel (luminance) which is enough for inverting a B&W-video. Note that you have do the same for the R-, G- and B- channels if you also want to invert the colors.