how to replace a string?

{
  string s1="palle";
  string s2=string.empty;
  s2=s1.Replace('l','z');
response.write(s2);
}

Somple code:

public class test

{

protected void page_load(-,-)

{

  string s1="palle";

 s1=s1+"tech";

StringBuilder sb =new StringBuilder();

sb.Append("palle");

sb.Appen("Tech");

}

}