Unscrew the screw, pull brush off, throw both in trash and spit on them!
Unscrew the two screws on either side of the motor! Pull on motor till it comes out! Dispose of!
The motor will slide right into the empty position, it is super easy. However you will need to remove the new brush from the motor so as to reinstall the bottom plate. After that reinstall the bottom plate then the brush again.
Play with the dogs that are you reason for buying a Roomba!