Use a suitable small tool to depress each of the small tabs on the brush cap (yellow arrows) while applying gentle pressure on the opposite end of the brush itself. These tabs seat into the rectangular openings (red arrow) on the brush holder. You should press on the exposed end the brush that contacts the commutator surface.

If it is stubborn you may use some appropriate solvent to help get it moving. Next, used a small piece of emory cloth and solvent to clean the inside of the brush holder. Followed up by blowing out with compressed air. Should look like new now.

Now clean the carbon brush itself but using a fine brush and solvent. Before reassembling brush holder assembly ensure the brush moves freely in the holder as it should. Note that there is a groove on the bottom of the brush that must be inserted into the bottom of the holder as shown in the picture. The brush can bind if you put it in backwards.

Reinsert the brush starting from the end at the top of the photo. Make sure the yellow arrow indicated tabs are fully engaged in the red arrow indicated opening in the holder. The one on the bottom too! You can gently bend the tab arms to properly be retained if needed.

This is a good time to blow out all sanding dust inside the housing while you are in here! Now you are ready to reassemble in reverse order of the above steps.