Open up the side of the tower. Get a fan. Blast fan into computer. On a hot day, this can make the difference between running hot but working, and running hot and shutting itself off to prevent damage.
Make sure the fan is blowing
with any fans inside the case, so angle it properly. IF you don't, it places stress on the bearings in the fans and can cause premature bearing failure. Replacing fans is a pain in the ass
edit: My gaming PC, which is what I'm on now, has 9 fans. I still have the side off because of how hot it is in here. Download a temperature monitor like CoreTemp. It gives you the temperatures of your CPU(s). IF they get up towards 50c, that's pretty damn hot ofr a CPU.