A warning or sensation of heat in the device is the normal consequence of the CPU handling excessive data, which makes the battery warm. It is not harmful to the battery or device even if the device feels hot.
 

Use the following actions to make your device return to its normal temperature:
1)    It is suggested not to expose your device in sunshine for a long period of time;
2)    Close large power consumption applications, such as music, chatting applications, GPS application, 3D gaming, video streaming etc.;
3)    Reduce the brightness of the screen to lower the temperature.
4)    Avoid using your device while charging. 
5)    Avoid using a protective case that does not dissipate heat. Remove your protective case if your device heats up.