change value of xml node via textbox c#

So I have a xml file

<?xml version="1.0" encoding="utf-8" ?>
<customers>
  <customer id="1">
    <username>ef</username>
    <password>eefer</password>
  </customer>
  <customer id="2">
    <username>efewf</username>
    <password>ewfwf</password>
  </customer>
</customers>

From a login page a customer is logged into a page and their session is stored on index page as a string like, string username = Session["username"].ToString(); and displayed on the page.

How can I change customer's password? So far I have this form:

<asp:TextBox ID="OldPass" runat="server"></asp:TextBox>
<asp:TextBox ID="NewPass" runat="server"></asp:TextBox>
<asp:TextBox ID="ConfPass" runat="server"></asp:TextBox>
<asp:Button ID="change" runat="server" Text="Button" OnClick="change_Click" />
<asp:Label ID="msg" runat="server" Text=""></asp:Label>

This code:

 protected void change_Click(object sender, EventArgs e)
 {
      XmlDocument doc = new XmlDocument();                    
      doc.Load(Server.MapPath("customers.xml")); 

      foreach (XmlNode node in doc.SelectNodes("/customers/customer/password"))
      {
        String password = node.SelectSingleNode("password").InnerText;

        if (...)
        {

        }
        else
        {

        }
     }
 }

How could I change or update from an old password of the customer to new one based on the logged in session maybe? or their id? thanks