The extracted motor and its wiring are shown in the first image, with the plastic carbon brush housings highlighted with red arrows. One screw holds the brush housing in place (removed in image). Often there are two adjacent screw holes, but I think the unused hole is a backup in case the thread is damaged in the first hole.

With the above screw removed, the brush housings can be eased out (note location for reassembly ie the blue wired brush housing is adjacent to the red wire.

With the brush housing removed, inspect the commutator 2nd and 3rd image). It may be slightly blackened or damaged (green arrow in second image). This can be gently cleaned as required eg using a rubber pencil eraser or a fine 1200 grit abrasive paper.

Blow out any abrasive grit and carbon dust, especially away from the bearings at either end of the rotor shaft.

The third image, shows a cleaned commutator which is now copper coloured, but still shows slight damage to one of the commutator contacts, but hopefully this damage should not be a problem, other than possibly wearing down the new brushes slightly faster.