If it is not got obvious screws on the case- it is a non serviceable piece of equipment.
It will either have clips that snap into place when the two case halves are assembled at the factory- or be chemically sealed together (glued).
Only one I ever tried to use- had the cable wrapped around the body and thrown as far into the surrounding trees as I could muster.
Compared to a good sharp hand file- I find these about as good as teats on a Bull, but other people swear by them.
Each to their own, but I would suggest rather than new brushes you might be looking at a new complete unit.