Answer:
Using the wingadium leviosa spell to bring one of the stone benchs to the red cross in front the climbing tree that connects the ground to the roof. After that, continuing to use the same spell to bring the second stone bench to the same place and put it on the first one bench. Then, Harry will climbs up to them to get to the roof.....then use reparo spell to repair the broken roof to reach the Fred & Geogre's box. After that, using the wingadium Leviosa to move the box down to the big Red cross in front of the foutain, then use Reducto spell tho mash the box. This box will change the courtyard into the swamp and that is what you help Iorna ! Enjoy !