Untitled

void UpdateXmlProcedure()
{
    XmlDocument doc = new XmlDocument();
    doc.LoadXml(@"
<book genre='novel' ISBN='1-861001-57-5'>
    <title>Pride And Prejudice</title>
    <note>
        <to>Tove</to>
        <from>Jani</from>
        <heading>Reminder</heading>
        <body>Don't forget me this weekend!</body>
        <住址>台北市</住址>
    </note>
</book>");

    doc.SelectSingleNode(@"/book/note/from").InnerText = "我是好新的值";
    doc.SelectSingleNode(@"//住址").InnerText = "臺灣省";

    Console.Write(doc.OuterXml); // 結果
}

=========================
doc.OuterXml
"<book genre=\"novel\" ISBN=\"1-861001-57-5\"><title>Pride And Prejudice</title><note><to>Tove</to><from>我是好新的值</from><heading>Reminder</heading><body>Don't forget me this weekend!</body><住址>臺灣省</住址></note></book>"