Wear loose cotton clothes (long-sleeved if stepping out in the hot sun)
Keep yourself hydrated.
Avoid soft/ aerated drinks/ soda & coffee as they'll dehydrate you. Stick to cool water, ice chips, fresh lemonade made with water, fresh fruit juices, melons, spinach-cucumber-onion-tomato salads, yoghurt,
Eat light.
Stick to well-ventilated rooms with good air-circulation (fans help)
Cold water showers to cool down
Sweating is good. It'll cool you down. This is also why Indians eat spicy food and drink hot tea even in hottest summer. Get sweaty then take a quick cold-water rinse.
If you have to step outside in the hot sun, umbrella, hats, caps etc are your friends.
Wet towel on the back of the neck for a quick cool down.
ETA: When it gets so hot that we lose our appetite, then our go-to meal is to mix up cooled cooked rice with unsweetened yoghurt and a pinch of salt. its variously called yoghurt rice/ curd rice/ thayir saadam / dahi bhaath / dahi chaawal . This is an easy to make & easy to diges meal that is guaranteed to cool a person down.
thayir = dahi = curd = yoghurt
saada = bhaath = chaawal = cooked rice