This is one qualm, that I always come across during summer, and winter. It is maintain ambient temperature indoors during summer or winter. Although there are forced machines such as Air conditioners to cool down or heaters to warm up. It is a good idea to keep these interventions to a minimum and opt for sustainable, simple and long term measures.
How to be cool?
Hot air stays at the top and cold air at the bottom. Exhaust fan can accelerate removing hot air from the room faster. During cooler times of the day, morning and evening keep the windows open to allow free flow of air.
Cool your body
Drink water. Wash face with cold water a couple of times during the day. Run normal or cold water over you hand for a few minutes.
Involve in some group activity
Group activities such as indoor games, or work takes away our attention from heat. You feel more heat and uncomfortable when you are alone.
Switch off appliances when not in use
Your computer, oven, or any appliance if it is not in use, it is better to turn it off. First to save energy and not to create extra heat in the room. Also, it is better to operate heat producing appliances in the cooler times of the day.
Plant trees and build a small garden
One sustainable and important measure is to plants with more leaves and the ones which grows faster and gives some shade. They will keep life healthy and comfortable no matter the season. A garden no matter how small can help in reducing the heat by absorbing the sunlight and keeping the surroundings at least cooler than it is without plants.