The most basic operation for editing character bitmaps is to change black pixels to white or the reverse; put another way, inverting the pixel the mouse is on. You do this by clicking the third mouse button.
Technically, this is just a special case of changing more than one pixel: when you press the third button, the current pixel inverts; then, as you move the mouse, the pixels it touches change to the color the first pixel changed to. Thus, if you press the third button on a white pixel, the mouse effectively becomes a "black pen" (until you release the button).
13.1.2.1 Selections Marking pixel regions for later operations.13.1.2.2 Enlarging the bitmap Give yourself more room at the edges.