How can i able to avoid over heat in my iphone

If your phone is overheating (warning on the screen and phone shutting down) even if the environment is within the appropriate temperature range, there is something wrong with your phone. You should turn it off, remove it from the charger and contact Apple.

Genius Bar Reservation and Apple Support Options - Apple

If it's just getting hot while you're playing a game, stop playing for a while and let it cool off.

Also, when you're charging the phone, it's normal for it to get warm. However, you should be sure that the phone is not sitting on a soft surface or covered while charging. In other words, don't put it under your pillow when you charge it.