First, remove the screws holding the middle drive cage from either side of the case. Pull the cage gently out through the case, taking care not to detach the fan wire. Then separate the cage from the front fan by undoing the latches (orange arrows) with a screwdriver.
Then the fan portion can be re-inserted through the front, and fastened using the screws that were previously removed.
The orange arrows show the latches that can be undone to remove the HDD cage. This allows the HDD cage to be separated from the front fan unit. Then reinsert the fan unit, and screw it to the sides of the case through the screw holes indicated by the green arrows.