String Replace and Regex String Replace Not Working

Hi all I am trying to replace a string with another string inside of an input string and it is not working, the following is my code:
string oldContent = "<p>\r\n  pbubp jjnl\r\n  </p>\r\n  <img src=\"../images/One.png\" />\r\n  </div>\r\n\r\n  </body>\r\n</html>\r\n\r\n""; string oldSource = "<img src=\"../images/One.png\">"; string newSource = "<img src=\"../images/Six.png\" alt=\"KEY\">"; string newContent = Regex.Replace(oldContent,oldSource,newSource); I also tried the following but it didn't work.
string newContent = oldContent.Replace(oldSource, newSource); Please point out the mistakes I made thanks.