Replacing substring

using System;/*from   j a  v a  2s. co m*/

class Sample
{
    public static void Main()
    {
        string s = "java2s.com";

        s = s.Replace("java", "JAVA");

        Console.WriteLine(s);

    }
}

using System;  /*from j a v a 2s .  c o m*/
  
class MainClass {  
  public static void Main() {  
    string str = "This is a test"; 
 
    Console.WriteLine("Original string: " + str); 
     
    // Replace characters 
    str = str.Replace('a', 'X'); 
    Console.WriteLine(str); 
 
  } 
}