Something about XML XPATH

path selection
Root/Channel/Program

path selection with attribute
Root/Channel/Program/@Title

path selection with filtering
Root/Channel/Program[@Title='Cinema' and @Time='05:20PM']
Root/Channel/Program[@Title='Cinema' or @Time>'05:20PM']


Root/Channel[@ID>10]/Program


EXAMPLE:
Suppose an xml

<?xml version="1.0" encoding="utf-8"?>

<Roles>

  <Role RoleID="1">

    <RoleName>administrator</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="2">

    <RoleName>user</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="3">

    <RoleName>Data Manager</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="4">

    <RoleName>Report Manager</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="5">

    <RoleName>customer</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="6">

    <RoleName>clerck</RoleName>

    <Status>Active</Status>

  </Role>

  <Role RoleID="7">

    <RoleName>Employee</RoleName>

    <Status>Active</Status>

  </Role>

 </Roles>

To Update an User 

private void UpdateRole(int RoleID,string RoleName,string Status)

        {

            string XmlFilePath = Server.MapPath("~/Roles.xml");

            XPathNavigator nav;

            XmlDocument docNav;

           

            docNav = new XmlDocument();

            try

            {

                docNav.Load(XmlFilePath);

                nav = docNav.CreateNavigator();

                nav.SelectSingleNode(string.Format("Roles/Role[@RoleID='{0}']/RoleName",RoleID)).SetValue(RoleName);

                nav.SelectSingleNode(string.Format("Roles/Role[@RoleID='{0}']/Status", RoleID)).SetValue(Status);

                docNav.Save(XmlFilePath);

            }

            catch (Exception ex)

            {

                throw ex;

            }

        }

And add an user node 

private void InsertRole(int RoleID, string RoleName, string Status)

        {

            try

            {

                string XmlFilePath = Server.MapPath("~/Roles.xml");

                XmlDocument docNav = new XmlDocument();

                docNav.Load(XmlFilePath);

                XmlNode NodeParent = docNav.SelectSingleNode("Roles");

                XmlNode newChild = docNav.CreateNode(XmlNodeType.Element, "Role", null);

                XmlAttribute xa = docNav.CreateAttribute("RoleID");

                xa.Value = RoleID.ToString();

                newChild.InnerXml = string.Format("<RoleName>{0}</RoleName><Status>{1}</Status>", RoleName, Status);

                newChild.Attributes.Append(xa);

                NodeParent.AppendChild(newChild);

                docNav.Save(XmlFilePath);

            }

            catch (Exception ex)

            {

                throw ex;

            }

        }