What if the screen keeps showing the temperature warning again and againg
my phone is showing temperature warning sign again and again, how to deal with it?
We can do nothing for you remotely, except suggesting to read this support guide:
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support
If you get no help, make an appointment at your most convenient Apple Store or Authorized Service Provider and have your iphone checked. They will give you all the available options:
Apple - Find Locations - Location Selector
Regards
Giulio