Our function will simply be the inverse of the given one: celsius and kelvin degrees differ by 273.15. You add this number is you compute kelvin from celsius, and you subtract this number if you're going the other way around (which is what we're doing):
#include
double KelvinToCelsius(double valueKelvin ) {
double valueCelsius = 0.0;
valueCelsius = valueKelvin - 273.15;
return valueCelsius ;
}