There are tons of tips about that if you make a quick search on the forum. Generally, it is advised to feed a lot of slightly damp leaves and to leave a cool place where the rabbit can rest - in my parents summer house, we have marble chimneys and my rabbits always spend the entire summer laying on the cool marble in front of them ^^. Stone works too. You can freeze a plastic bottle filled with water and put a towel around it (to avoid thermal choc) so your rabbit can lay against it. Of course, make sure Iris has fresh (not freezing! it would hurt her stomach) water to drink. If it gets really hot, you can wet her ears with some cold / tepid water as rabbits regulate their temperature with their ears.