Answer 1

Maybe it will help ...
#include <iostream>
#include <string>
using namespace std;

int main()
{
string st1("It's my first program");
string st2("your second program");
string st3("third ");

cout << "st1: " << st1 << endl;
cout << "st2: " << st2 <<endl;

cout << "replace 19 characters in st1 with st2:"<<endl;
st1.replace(5, 19, st2);
cout << st1 << endl;

cout<<"replace 6 characters in st2 with st3:"<<endl;

st2.replace(5, 6, st3);
cout<<st2<<endl;
return 0;
}
Output->
st1: It's my first program
st2: your second program replace 19 characters in st1 with st2: It's your second program
replace 6 characters in st2 with st3:
your third program

but for the replacement between st2 and st3 we should add a space after "third" i mean like this "third " ...because ...the word second has 6 characters ...so we put a space to make st3 6 characters.