<?xml version="1.0" encoding="utf-8"?>
<net_lover>
  <sitename><![CDATA[<h2>.NET 开发</h2>]]></sitename>
  <siteurl>https://www.ogeek.net</siteurl>
</net_lover>

System.Xml.XmlDocument d = new System.Xml.XmlDocument();
d.Load(Server.MapPath("~/1.xml"));
System.Xml.XmlCDataSection n = d.SelectSingleNode("/net_lover/sitename").FirstChild as System.Xml.XmlCDataSection;
n.Value = "<h1>.NET 开发</h1>";
d.Save(Server.MapPath("~/2.xml"));