Configuration configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None); 
configuration.AppSettings.Settings["節點名稱"].Value ="0"; 
configuration.Save(ConfigurationSaveMode.Modified);  

//獲取config路徑
   string path = System.Windows.Forms.Application.ExecutablePath + ".config";
   XDocument doc = XDocument.Load(path);
   //查找所有節點
   IEnumerable<XElement> element = doc.Element("configuration").Element("appSettings").Elements();
   //遍歷節點
   foreach (XElement item in element)
   {
       if (item.Attribute("key") != null && item.Attribute("key").Value == "節點名稱")
       {
           if (item.Attribute("value") != null)
           {
  item.Attribute("value").SetValue(DateTime.Now.ToString("d"));
           }
       }
   }
   //保存
   doc.Save(path);