The most important rule is to drink enough clean drinking water so that the body is not dehydrated. Tea, juices, kvass and other beverages will not give this effect and you can not replace water with them.
Food in hot weather should be as light as possible. The body is hard at high temperatures, so it is very important not to include heavy-to-digest food in your diet. If you don't have an appetite, you don't have to force yourself to eat! But you must drink!