One way to be cool when it's hot is to be underground somewhere

 In other words if you have enough air to breathe that is fresh enough to sustain you it isn't going to heat up unless it has sun beating down on it once you go below 2 to 5 feet or so wherever you are on earth. I have tested this out in the desert for example, by digging down through the looser sandstone under the earth when it's hot outside and it was much cooler in a man made or natural cave than outside as long as you have enough fresh air and there is no danger of this man made or already existing cave of burying you alive.

So, I envision more and more people where it gets really hot during the day living their lives mostly at night where it will be cool enough to go outside of a cave (man made or otherwise).

Another example is underground houses. Now this is a great idea but usually very cement intensive to do this. However, one way is just to bring the dirt up on top of your cement home and grow a lawn there so it doesn't erode during a rain storm. You can build a house where on two sides it goes up from the ground to the roof so you can literally mow your lawn by starting on one side of your house going up over the roof and down to the other side of the roof mowing your lawn. Then the other two sides of the house look like a normal house except for the roof. However, you have to likely use a lot of cement including upon your roof to make this work and maybe you might use pool paint to seal it so you don't have leaks of water coming down through the ceiling too for when it rains.

Anyway people are going to find whatever ways they can find to survive this heat and the ones that don't do this just won't be around anymore.

Because like I said when you get too hot your brain stops working and you become incapable of saving yourself. So, someone else has to save you from heat Stroke unless you can somehow get to a cooler place to shelter of some kind so you don't die or just go jump in a cool lake or river to survive if it is daytime.

But, remember some places in the U.S. there are alligators and people seem to be getting eaten more lately and I think it is likely people who aren't used to having  alligators in rivers and streams.

Also, with Global Climate changes alligators are slowly going to move more north just like many other creatures will as the temperatures rise everywhere on earth.

Eventually, Siberia, Northern Canada and Antarctica all will be colonized more as the snow and ice all melts away.

Then eventually everyone will likely have to live underground (at least during the day) to survive most places here on earth during the Summers and then the spring and fall eventually too.