Almost every smartphone is facing this problem due to several reasons. I am listing down the most common of them along with its cure. If you don't find any cure even after following them, it is recommended that you visit a service center and seek their help.
Overloading with Apps
If you have loaded your phone with too many apps, the result is overheating, as most of them are running in the background, even if you have come out of the screen. So you have to go to the Settings and find out the Running Apps from there. Once you find the list, you have to manually stop them.
Too long use of media files
If you have watched videos, movies or played music file for too long continuously, it would automatically result in heating up your phone. So, after watching or playing these files, give some breathing time to your phone, by keeping it in long standby mode for an hour. You can do this on regular basis while sleeping, as the alarm would be still on, even at this mode.
Rise of atmospheric temperature
Its hot summer wave flowing across the country. Everything is heated up, our phones are no exception. Try to keep it in a comparatively cool place, where direct sunlight is not reaching the phone. It would help you a bit.
Use Battery Saver Apps
As Timmappa has suggested, you can try these battery saving apps to cool down your ohone which are available in the play store. These apps generally take care of many issues like stopping unnecessary apps in the background and keeping the phone processor in an off and on mode.
Reduce the Display Light
By reducing the Display light you would be solving a row of things, your battery life, your eyes and this problem of overheating.
Reduce the Vibration Functionality
If you are putting your phone on ringtone, you don't need the vibration mode along with it. You can use it, only when you want it to be silent, and yet want your notifications on. This would save on the phone battery and its functionality. as a result it would cool down your phone.
Hope these helps.