How to modify existing XML file with XmlDocument a

I already implemented to create the XML file below with XmlTextWriter when application initialization.

And know I don't know how to update the childNode id value with XmlDocument & XmlNode.

Is there some property to update the id value? I tried InnerText but failed. thank you.

<?xml version="1.0" encoding="UTF-8"?>
<Equipment xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <License licenseId="" licensePath=""/>
  <DataCollections>
    <GroupAIDs>
      <AID id="100">
        <Variable id="200"/>
        <Variable id="201"/>
      </AID>
      <AID id="">
        <Variable id="205"/>
      </AID>
      <AID id="102"/>
    </GroupAIDs>
    <GroupBIDs>
      <BID id="2000">
        <AID id="100"/>
      </BID>
      <BID id="2001">
        <AID id="101"/>
        <AID id="102"/>
      </BID>
    </GroupBIDs>
    <GroupCIDs>
      <BID id="8"/>
      <BID id="9"/>
      <BID id="10"/>
    </GroupCIDs>
  </DataCollections>
</Equipment>

You need to do something like this:

// instantiate XmlDocument and load XML from file
XmlDocument doc = new XmlDocument();
doc.Load(@"D:\test.xml");

// get a list of nodes - in this case, I'm selecting all <AID> nodes under
// the <GroupAIDs> node - change to suit your needs
XmlNodeList aNodes = doc.SelectNodes("/Equipment/DataCollections/GroupAIDs/AID");

// loop through all AID nodes
foreach (XmlNode aNode in aNodes)
{
   // grab the "id" attribute
   XmlAttribute idAttribute = aNode.Attributes["id"];

   // check if that attribute even exists...
   if (idAttribute != null)
   {
      // if yes - read its current value
      string currentValue = idAttribute.Value;

      // here, you can now decide what to do - for demo purposes,
      // I just set the ID value to a fixed value if it was empty before
      if (string.IsNullOrEmpty(currentValue))
      {
         idAttribute.Value = "515";
      }
   }
}

// save the XmlDocument back to disk
doc.Save(@"D:\test2.xml");