Символьные массивы.

string replaceChar(string str, char ch1, char ch2) {
  for (int i = 0; i < str.length(); ++i) {
    if (str[i] == ch1)
      str[i] = ch2;
  }
 
  return str;
}

#include <string>
#include <algorithm>
 
std::string s = "03/02";
std::swap(s[1], s[4]);

#include <iostream>
using namespace std;
 
int main()
{
  char s[80]="In a string of 80 characters in the word WHELI swap the letters I and E",*p;
 
 
  p=strstr(s,"WHELI");
  p=strpbrk(p,"E");
  *p='I';
  *p++;
  p=strpbrk(p,"I");
  *p='E';
 
  cout<<s<<'\n';
  system("pause");
  return 0;
}

#include <iostream>
#include<cstdlib>
#define size 80
 
using namespace std;
 
int main()
{
system("chcp 1251 > nul");
char*arr=new char[size];
cin>>arr;
char*temp=new char[size];
size_t i=0;
 
while(arr[i]!='\0')
{
    if(arr[i]=='E')
        temp[i]='I';
    else if(arr[i]=='I')
         temp[i]='E';
    else
         temp[i]=arr[i];
    ++i;
}
 
delete[]arr;
 
arr=temp;
 
temp=nullptr;
 
cout<<arr<<endl;
 
delete[]arr;
 
arr=nullptr;
system("pause > nul");
return 0;
}

#include <iostream>
using namespace std;
 
int main()
{
  char s[80]="In a string of 80 characters in the word WHELI swap the letters I and E",*p;
 
 
  p=strstr(s,"WHELI");
  p=strpbrk(p,"E");
  *p='I';
  *p++;
  p=strpbrk(p,"I");
  *p='E';
 
  cout<<s<<'\n';
  system("pause");
  return 0;
}