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;
}
}
}
int main(){
char c1[]="Eevee";
interchangeCase(c1,'e');
printf("%s\n",c1);
char c2[]="Eevee";
interchangeCase(c2,'E');
printf("%s\n",c2);
}
Explanation: