The warning is a normal recommendation message and not harmful to battery or phone, phone will start recharging once returning to normal temperature.
You can follow below actions to make phone return to normal temperature:
- It is suggested not to expose your phone in the sunshine for long periods of time;
- Close large power consumption applications, such as music , chatting applications, GPS application, 3D gaming, video streaming etc.;
- Reduce the brightness of screen to lower the temperature