ASP.NET

string str = System.IO.File.ReadAllText(filepath);
str.Replace("<!-- {PLACEHOLDER} -->", "some text to put into the HTML file");

System.IO.File.WriteAllText(savepath);

string str = System.IO.File.ReadAllText(filepath);
str.Replace("</body>", "<script type=\"text/javascript\">alert('script inserted');</script></body>");

System.IO.File.WriteAllText(savepath);

string str = string.Empty;
using (System.IO.TextReader reader = new System.IO.StreamReader(System.IO.File.Open(path, System.IO.FileMode.Open, System.IO.FileAccess.Read)))
{
  str = reader.ReadToEnd();
  reader.Close();
}

str = str.Replace("</body>", "<script>// blah blah</script></body>");

using (System.IO.TextWriter writer = new System.IO.StreamWriter(System.IO.File.Create(path)))
{
  writer.Write(str);
  writer.Flush();
  writer.Close();
}