XML : replacing value of XML file in javascript

I load an XML document using XMLHTTPRequest in my js file. the XML file is like below:

    <?xml version="1.0" encoding="utf-8"?>    <RMFSFile xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">    <Host>www.example.com</Host>    <Port>8888</Port>       <Uri>www.example.com</Uri>  <Path>       <HD>          <UNC>path1</UNC>      </HD>      <SD>         <UNC>path2</UNC>      </SD>  </Path>    

I am now trying to first select Path1, and Path2 nodes, and then switch the value of path1 with the value of path2. I select the nodes like below:

   var firstNode = xmlFile.querySelector('Path>HD>UNC'); \\ I can get the node successfully.    var secondNode= xml.querySelector('Path>SD>UNC');  \\ I can get the node successfully.     

However, when I try to replace the values by using the following code:

      xmlFile.replaceChild(secondNode,firstNode) // secondNode, is the new value for first node.    

I get an error says,"An attempt was made to reference a Node in a context where it does not exist". Any Idea where I am doing it wrong?