XML : Change value of element in xml file which is based on raml21.xsd schema in asp.net

below is xml file which I want to update (for example - change dateTime attribute in log element).

  <?xml version="1.0" encoding="UTF-8" standalone="no"?>  <raml xmlns="raml21.xsd" version="2.1">  <cmData id="3221225472" scope="all" type="plan">  <header>  <log action="created" appInfo="Nokia BTS Site Manager" appVersion="FL16" dateTime="2016-09-23T17:59:59" user="BTSSM"/>  </header>  /cmData>  </raml>    

Here is my code:

  filePath = openFileDialogXml.FileName;  XmlDocument document = new XmlDocument();  document.Load(filePath);  XmlElement node1 = document.SelectSingleNode("/header/log") as XmlElement;  if (node1 != null)  {  node1.Attributes[3].Value = "test";  }  document.Save(filePath);    

But, the problem is: node1 is alwas null. I now the problem cause this line in xml file:

  raml xmlns="raml21.xsd" version="2.1"    

but, i can't find solution.

Thanks in advance..