how to save xml file with ajax and javascript

I am trying to add an element to an xml file. I have checked the program with debugger and I saw it really adds the element to the xml file but when I stop the running the file didn't saved any changes. here is the javascript file:

var xmlhttp = LoadXMLHttp();
var xmlDoc=LoadXMLDoc("XMLFile.xml");;
function LoadXMLHttp() {
    var xmlHttp;
    if (window.XMLHttpRequest)
        xmlHttp = new XMLHttpRequest();
    else
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
    return xmlHttp;
}
function LoadXMLDoc(FileName) {
    xmlhttp.open("GET", FileName, false);
    xmlhttp.send(null);
    return xmlhttp.responseXML;
}
function CreateXmlElement() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        newMassageElement = xmlDoc.createElement("massage");
        newTextElement = xmlDoc.createElement("text");
        newText = xmlDoc.createTextNode("I am fine");
        newTextElement.appendChild(newText);
        newMassageElement.appendChild(newTextElement);
        x = xmlDoc.documentElement;
        x.appendChild(newMassageElement);
    }
}
function AddXMLElement() {
    xmlhttp.open("POST", "Default.aspx", true);
    xmlhttp.setRequestHeader("Accept", "text/xml");
    xmlhttp.onreadystatechange = CreateXmlElement;
    xmlhttp.send(xmlDoc);
}

And here is the xml file:

<?xml version="1.0" encoding="utf-8" ?>
<conversation>
  <massage>
    <text>Hi</text>
  </massage>
  <massage>
    <text>How are you?</text>
  </massage>
</conversation>

B.T.W:

1) I dont know jquary or php but I do know asp.net

2) If I change the open url to "XMLFile.xml" I get an error massage that says:" method not allowed"

3) I have a button that that actives the AddXMLElement() function