Ice!

Get or make an insulated water bottle. Fill with ice cubes, then water. When you start feeling the heat, fill your mouth about half full of ice water and hold it there for 30 seconds, then swallow a tiny bit at a time to cool inside your throat. Mouth and throat are loaded with blood vessels that will cool a large part of your upper body, neck, and head. Wait five minutes. Repeat. If you run out of ice water, add another bottle to your collection, only freeze this one overnight and save for last.

Wear a hat or helmet to shade your scalp and head. Radiating sun heats you up like the surface of Mercury. I wear full zip bike jerseys made of very light fabric, or very light long sleeve dress shirts from Goodwill. Cotton will stay damp but is much cooler than synthetics. I never button or zip a shirt or jersey at all June through September in South Louisiana. Like riding under a shade tarp. Try to avoid backpacks.

Spend time in the heat other than on your bike. Stay acclimated to the heat as much as possible this way.