I don't have a manual either but here's what I just did.
The top cover of the sweeper unit looks like a single piece of plastic but it isn't; the top part that covers light is separate. Pull/squeeze/jiggle this piece to remove it.
At this point I'd recommend removing the light bulb to avoid breaking it when removing/replacing the main cover.
With the light cover removed, you'll see two screws; remove them.
Now flip the unit upside-down. On either side in between the sides of the outer cover and the body you'll see little tabs; on mine the underside even says something like "pry with screwdriver to remove." Slide a straight screwdriver between the body and outer cover and gently lever the body up out of the cover on both sides.
Once both tabs are free, turn the entire unit back over and remove the outer cover. The roller brush lifts out to replace the belt.
Andrew
December 2007