Answer:
#include <stdio.h>
void interchangeCase(char phrase[],char c){
 for(int i=0;phrase[i]!='\0';i++){
   if(phrase[i]==c){
     if(phrase[i]>='A' && phrase[i]<='Z')
       phrase[i]+=32;
     else
       phrase[i]-=32;   Â
   }
 }
}
int main(){
 char c1[]="Eevee";
 interchangeCase(c1,'e');
 printf("%s\n",c1);
 char c2[]="Eevee";
 interchangeCase(c2,'E');
 printf("%s\n",c2);  Â
}
Explanation: