In step 4, the screw on top of the housing has an unusual head. It's similar to a Phillips screw, but it only has 3 grooves. A small flat blade screw driver can unscrew it.

When you pry off the cover of the gear box, some gears may pop out, so do this on a work surface where they won't get lost.

You may find that the brush has stopped rotating because the inside of the gear box is full of lint.