To remove the bottom panel and gain access to the bottom fans, start by pulling out the bottom filter from the rear and lift off the outer case cover.
When that's done, release the 4 screws securing the bottom panel to the case:
When these are out, you can remove the bottom panel. It is however clipped in to the case by two hooks in the center of both sides of the panel:
This means that in order to get it off, you lift the front part of the panel and slide it out this way:
