If it's during the week, I ride at 5 AM. If it's on the weekend, and it's a longer ride, I try to make it in a shady area, which is pretty easy to do around here, as well as go early. However, sometimes, it's a "planned" ride and I deal with it by riding slower, dousing myself with water, and eating lots of Shot Blocks and plenty of Nuun.
If it's very hot, humid, and there's an air quality alert, which happens sometimes (not too often), I go to the gym, or take a brisk walk.
For the first 5-6 years that I rode, I would go to spin class during a heat wave. I haven't done that in a long time. If I feel like it's dangerous, I am more apt to go into the woods and hike or trail run.
For me, anything over 75 or so feels hot. Being in the bright sun bothers me, because most of the riding I do is in very shady areas. Like Oak says, I do better if I can acclimate, but here in the east, the weather is not consistently very hot. Three years ago, we had a very hot summer. I was fine, because I had just done my tour in Spain, where 4/6 days it was about 100 degrees when we were riding. That was not fun.