When the sun is high and it is not cloudy... It could be great if it would not be so warm..
The obvious solution is to find a shadow. Partially that helps. However the air could still be warm. So it could be hard even not under the sun.
If in the city, the nearest cafe/library/shopping mall/railway station is the oasis because they have air conditioner. The cost to get to the cafe is as low as the price of the cup of coffee.
If outside of the city, the nature also have some solutions. Water (lake, river, sea) is a great relief even if the water per se is relatively warm. This works great especially if we stay in the water for a longer time (5-10 minutes and more).
The ground (earth) is usually cold. So just laying down on the earth makes our body colder.
Since the earth is cold, basement is also cold. If we have time and stay outside for multiple days, we can make a basement by ourselves. Dig a hole, watch "how-to" YouTube, cover it with sticks and leafs.
If driving and sitting in the car, and car air conditioner is not working.. That is not good.. However there are still solutions even though not 100%. Multiple windows of the car should be open (not only one). This will allow air to go through and circulate. Another option is just to spray ourselves with the water bottle, starting from the head (hairs). Hairs will become dry longer, so they can hold water for a longer time.