There are three screws that hold the shell together.
The rear (phillips) screws are in the rear wheel wells. The front screw is also accessed from the bottom, in the front where the power plugs in for the brush. After the screws are out, carefully split the case. The bumper might be pretty snug, holding the shell halves together. Once the front is loose, it should pop apart
This is how my CXL Tristar came apart.
Bill
July 2008