public static void Main()
string str = "w3resource";
str = str.Remove(pos, 1).Insert(pos, replacement.ToString());
str = str.Remove(pos2, 1).Insert(pos2, replacement2.ToString());
str3 = str3.Remove(pos3, 1).Insert(pos3, replacement3.ToString());
str4 = str3.Remove(pos4, 1).Insert(pos4, replacement4.ToString());