seekg( ) moves the pointer offset number of characters from the specified origin : file pointer « File Stream « C++ Tutorial

1. C++ Tutorial
2. File Stream
3. file pointer

/*
It must be one of these three values:

ios::beg         Beginning-of-file

ios::cur         Current location

ios::end         End-of-file
*/

#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
   
int main(int argc, char *argv[])
{
  if(argc!=4) {
    cout << "Usage: CHANGE <filename> <character> <char>\n";
    return 1;
  }
   
  fstream out(argv[1], ios::in | ios::out | ios::binary);
  if(!out) {
    cout << "Cannot open file.";
    return 1;
  }
   
  out.seekp(atoi(argv[2]), ios::beg);
   
  out.put(*argv[3]);
  out.close();
   
  return 0;
}