A frozen 2 liter bottle of water is appreciated as they can lay up against it. Some folks put a floor tile in the freezer and let them lay on it to cool off.
You can wet the ears with cool water making sure not to get water in their ears. Rabbits release body heat through their ears, it is the only way they can.
If your rabbit is damp around their nostrils and panting, get them to a cooler area even if it means bringing them into a cool room in the house for a few hours during the hottest part of the day.
If a fan can be hooked up that helps but don't have it blowing directly on high at them.
Plenty of clean water should be available at ALL times. If using a water bottle make sure it is clean of algae and that the tip or ball is working properly. A dish of water can quench a thirst even quicker.