Update Existing Xml file

//Xml File

?xml version="1.0" encoding="utf-8"?>
CategoryList>
Category ID="01">
MainCategory>XML /MainCategory>
Description>This is a list my XML articles. /Description>
Active>true /Active>
/Category>
Category ID="02">
MainCategory>Asp.net /MainCategory>
Description>Subahsh /Description>
Active>True /Active>
/Category>
/CategoryList>


//aspx.cs

if (!IsPostBack)
{
XmlDocument xml = new XmlDocument();
string s = Server.MapPath("XMLfILE.xml");
string list = "/CategoryList/Category";
xml.Load(s);
XmlNodeList xn = xml.SelectNodes(list);
foreach (XmlNode eleSourceNode in xn)
{
string ID = eleSourceNode.Attributes["ID"].Value;
string author1 = eleSourceNode["MainCategory"].InnerText;
DropDownList1.Items.Add(new ListItem(author1, ID));
}


}

//DropDown SelectIndexChange

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load(Server.MapPath("XMLfILE.xml"));

XmlNodeList nodeList = xmlDoc.SelectNodes("/CategoryList/Category[@ID='"+DropDownList1.SelectedValue+"' ]");

TextBox2.Text = nodeList[0].ChildNodes[0].InnerText;
TextBox3.Text= nodeList[0].ChildNodes[1].InnerText;
TextBox4.Text = nodeList[0].ChildNodes[2].InnerText;

}

//Update Xml File

protected void btnUpdateXml_Click(object sender, EventArgs e)
{
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load(Server.MapPath("XMLfILE.xml"));

XmlNodeList nodeList = xmlDoc.SelectNodes("/CategoryList/Category[@ID='" + DropDownList1.SelectedValue + "' ]");

// update MainCategory
nodeList[0].ChildNodes[0].InnerText = TextBox2.Text.Trim();
// update Description
nodeList[0].ChildNodes[1].InnerText = TextBox3.Text.Trim();
// update Active
nodeList[0].ChildNodes[2].InnerText = TextBox4.Text.Trim();

// Don't forget to save the file
xmlDoc.Save(Server.MapPath("XMLfILE.xml"));
}