What can I do when my phone pops up a warning saying, “high temperature”?

The warning is the normal consequence of the CPU handling excessive data, which makes the battery warm. It is a recommendation message and not harmful to battery or phone even if the phone feels hot. You can follow below actions to make phone return to normal temperature:

1) It is suggested not to expose your phone in the sunshine for long time;

2) Close large power consumption applications, such as music , chatting applications, GPS application, 3D gaming, video streaming etc.;

3) Reduce the brightness of screen to lower the temperature.