This is usually the first thing you’ll want to do, and often the main issue that will be stopping the car from functioning properly. After years of abuse a fair bit of gunk can build up where the wheel axle meets the body of the car and the motor. Once you’ve got the body off, and dust cover removed, you’re going to need a very fine screwdriver or tweezers. You’re going to use this to scrape out all the gunk from around the axles, bit by bit until it’s all gone. Don’t think about using water and some kind of brush for this – water and this motor aren’t great together, probably not the end of the world, but you don’t want to get water in there, and you don’t want to blow any of this dust and gunk into there either, so just pick and scrape it away. The front wheels don’t normally have any problem, but check those out while you’re at it.
As far as the motor itself goes, you’ll want to try and clean that down as much as possible without taking it out if you don’t need to. So easiest way to do that is with some cotton buds sprayed with CRC (WD40 will also be ok for that) then just wipe away at it, all around until it’s as clean as you can get it.