The center chassis is aligned to the shell with guide pegs. These pegs will need to slide out of their slots to allow the chassis to be lifted out.

Grip the center chassis and pull it towards the green fan grille at the top of the shell, uncoupling the guide pegs from the shell.

Lift out the center chassis assembly to remove it from the shell.

During reassembly, make sure neither of the ribbon cables get pinched as you lower the center chassis into the shell.