如何更改XML节点值？

I've reviewed numerous posts on this topic and have tried to make adjustments to my code but can not seem to make it work, any help is greatly appreciated.

How do I update the value of cen_inst_units_z_name?

XML

<?xml version="1.0" encoding="UTF-8" ?> 
<xfa:data xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/">
    <form1>
          <cen_inst_units_z_name>Department of School</cen_inst_units_z_name>
    </form1>
</xfa:data>   

PHP

<?php 
    $doc = new DOMDocument();
    $doc->load('52500_data.xml');
    $xpath = new DOMXPath($doc);

    $parent = $doc->getElementsByTagName('form1')->item(0);
    $query = $xpath->query('cen_inst_units_z_name',$parent);

    //Checked to make sure I was pulling the correct node
    echo $query->item(0)->textContent;

    $query->item(0)->nodeValue = 'Test123'; 
    $doc->saveXML();
?>

I changed code to save a new document and got the result I was looking for:

$doc->save("newdoc.xml");