There are several steps to cleaning the motor:
Clean the chassis and speed control
An air compressor can be used to blow off the dust and dirt that has accumulated on the chassis and speed control. It’s best to work with a clean car and workspace to prevent crud from getting into the motor. If necessary, use an old toothbrush to remove stubborn dirt deposits.
Inspect the wiring
Check the speed control for frayed wires and damaged insulation; these can lead to radio interference and, worse, short-circuiting, which can burn up the motor and speed control. Repair damaged wires with heat-shrink tubing or liquid electrical tape.
Remove the motor
You do not need to unsolder the motor’s power leads or unplug the sensor harness, but doing so will give you more freedom to work. After removing the motor, wipe it with a rag to remove dust and dirt.
Disassemble the motor
Take the motor apart (this is quite easy to do) then you can wipe the individual parts of the motor.
Clean the bearings
Remove the rear bearing from the motor for cleaning. The front bearing on Novak brushless motors is glued to the front endbell and cannot be removed; leave the front bearing in place, and give it a good dousing with motor spray.
(Steps adapted from this link)
How to take the motor apart: (added per OP's request)
IMPORTANT: Before disassembling the motor, if your item carries a warranty, opening and attempting your own repair, will void whatever warranty you have.
First, get to know the parts of your brushless motor:
The parts needed/recommended for disassembly:
The first step involves in removing the circlip (E-ring), followed by the collar ring from the rear shaft using the pliers. Be cautious when removing the circlip because it might spring out. If you do not have any spares, do not lose it or bend it out of shape.
The second step is to pull and remove the rotor (the bell) from the stator (the body).
Notice that there are two bearings attached to the rear end and front end of the stator. The next step is to remove the bearings from the stator. Sometimes, the bearings are easy to remove, and some have a tight fit. Remove one of the bearings using barbeque skewer or any wooden stick to plug out the bearing from the stator. Usually, the front end bearing is easier to remove.
After you have managed to remove one of the bearings, use a pin punch tool to push the other bearing out from the stator.
Place the stator on the plastic wire spool / solder bobbin and carefully tap the pin punch tool using the hammer until the bearing drops out from the stator.
The next procedure is to remove the shaft from the rotor (the bell). Use the allen key to remove the set screw on the rotor.
The final step is to push the shaft out from the rotor using the similar pin punch tool technique implemented previously to remove the bearings.
Motor disassembly steps adapted from this article.