?xml version="1.0" encoding="utf-8"?>
net_lover>
  sitename>![CDATA[h2>.NET 開發/h2>]]>/sitename>
  siteurl>https://www.jb51.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 開發/h1>";
d.Save(Server.MapPath("~/2.xml"));