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");

You can just open a new file, write the string and the Content of the old file.

ifstream oldXML("path/to/old/xml");
ofstream newXML("path/to/new/xml");

newXML<<"<!DOCTYPE people SYSTEM \"new_xmll.dtd\">"<<endl;  //Write first line
newXML<<oldFile;    //Copy Content of old file

You can also use a XML library if you want to Change any XML values. I like tinyxml a lot

Since XML is text, any text editor will work fine. With XML, "data" can be just about anything in the file, including the structure, so what you will need to edit is entirely dependent on your application.

In general, however, anything non inside angle brackets (< and >) is data, and also attributes on the elements (the things in the angle brackets) is also often data.

You could use the XDocument class:

var doc = XDocument.Load("test.xml");
var address = doc.Root.Element("address");
if (address != null)
{
    address.Value = "new value";
}
doc.Save("test.xml");

Use the XmlDocument class. Then you can filter to the addresses with a call to GetElementsByTagName(string). From there, modification should be easy.

Well, If you want to update a node in XML, the XmlDocument is fine - you needn't use XmlTextWriter.

XmlDocument doc = new XmlDocument();
doc.Load("D:\\build.xml");
XmlNode root = doc.DocumentElement;
XmlNode myNode = root.SelectSingleNode("descendant::books");
myNode.Value = "blabla";
doc.Save("D:\\build.xml");

By design, XmlReader represents a "read-only forward-only" view of the document and cannot be used to update the content. Using the Load method of either XmlDocument, XDocument or XElement, will still cause the entire file to be read in to memory. (Under the hood, XDocument and XElement still use an XmlReader.) However, you can combine using a raw XmlReader and XElement together using the overloads of the Load method which take an XmlReader.

You don't describe your XML structure, but you would want to do something similar to this:

var reader = XmlReader.Create(@"file://c:\test.xml");  
var document = XElement.Load(reader);
document.Add(new XElement("branch", "leaves"));
document.Save("Tree.xml");

To find a specific node (for example, with a specific attribute value), you'd want to do something similar to this:

var node = document.Descendants("branch")
                   .SingleOrDefault(e => (string)e.Attribute("name") == "foo");