Remove a screw at the right hand end of the cover between the fan and the grill. (The screw at the other end was removed in removing the motherboard.)

Lift off this cover. Note that it's a snug fit. On refitting, ensure that the screw holes are correctly aligned with the pillars the screws scew into.

On reassembly, if you have removed the optical assembly and/or the fan, don't finally tighten the screws on these until you've refitted the fan/grill cover.