how to write in an existing xml file

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Hi Everyone,

I've used this code to read an existing xml file -

using (XmlReader reader = XmlReader.Create("XMLFile1.xml"))
{
  while (reader.Read())
  {
    switch (reader.NodeType)
    {
      case XmlNodeType.Element:
        if (reader.ReadToFollowing("title"))
        {
          output.AppendLine("Title: " + reader.ReadElementContentAsString());
          output.AppendLine();
        }
        if (reader.ReadToFollowing("first-name"))
        {
          output.AppendLine("First Name: " + reader.ReadElementContentAsString());
          output.AppendLine();
        }
        if (reader.ReadToFollowing("last-name"))
        {
          output.AppendLine("Last Name: " + reader.ReadElementContentAsString());
          output.AppendLine();
        }
        if (reader.ReadToFollowing("price"))
        {
          output.AppendLine("Price: " + reader.ReadElementContentAsString());
        }

        break;
    }
  }
  textblockTitle.Text = output.ToString();

  output.Remove(0, output.Length);
}

(where output is a StringBuilder object)

this is my XML file (XMLFile1.xml)

<!-- This is a sample XML document -->
<bookstore>
  <book genre="autobiography" publicationdate="1981-03-22" isbn="1-861003-11-0">
    <title>The Autobiography of Benjamin Franklin</title>
    <author>
      <first-name>Benjamin</first-name>
      <last-name>Franklin</last-name>
    </author>
    <price>8.99</price>
  </book>
</bookstore>

Now i want to write/append to this file,
how could i do this?
(XMLFile1.xml is an existing xml file.)