First follow the fan removal instructions. Loosen and then remove all eight screws as indicated here. Laying out a towel on the workspace before beginning helps keep screws from being lost if dropped.
When the screws have been removed swing the back off to the side and free the fans wiring harness from the opening in the grill
When replacing the Rear Cover, first feed the fan harness and connector through the same opening shown in the previous step. Hold the Rear Cover in place making sure that it is properly aligned all around its perimeter. Insert and loosely tighten all screws. When all screws are in place and Rear Cover is properly aligned, tighten the screws. Do not over-tighten! Snugged into place is sufficient.