you should be able to clean or change the brushes with the motor in place.
use electric motor cleaner and blow out with compressed air to dry out any residual moisture.
If you change brushes make sure the sensor wires (if any) are not rubbing any other components. Make sure the springs seat well.
After installing the brushes it is a best practice to use a brush seating stone to clean the armature and help seat the brushes to avoid any excess arcing. New brushes do not form to the armature as one might think so you should use a seating stone on the armature to clean the armature and help the brushes seat properly.
This may take 2 people to perform this, one to operate the pump circuit and the other to use the stone on the armature while the motor is running.
Do not get too carried away with cleaning and seating, it only takes a small amount of pressure and watching the armature, once it looks clean and the arc'ing is minimal is when you know your done.
There will always be a small amount of arc'ing due to the nature of the contact surface, you just don't want to see an excess amount of it.

You can get these seating stones from any motor servicing business or from the dealer.