How can i update xml file click on the button

[Jigar]

this is my function...plz help me.....
function upDate()
{
xmlhttp.open("GET","xmlf/emp.xml",false);
xmlhttp.send(null);
xmlDoc=xmlhttp.responseXML;
var x=xmlDoc.getElementsByTagName("Student");
for(i=0;i<x.length;i++)
{
if(document.getElementById("se").value == x[i].getElementsByTagName("Name")[0].childNodes[0].nodeValue){
var a = x[i].getElementsByTagName("Address")[0].childNodes[0].nodeValue = document.getElementById("address").value;
alert(x[i].getElementsByTagName("Address")[0].childNodes[0].nodeValue = document.getElementById("address").value);
alert("In");
}
}
alert("end");
}

[Jimmaay]

	var xmlhttp = new XMLHttpRequest(); // You must Initialize
	xmlhttp.open("GET","x.xml",false); // async is false, which is fine, but try async sometime
	xmlhttp.send(); // no need for null
	var x=xmlhttp.responseXML.getElementsByTagName("Student"); // only need one variable
	for(var i=0,j=x.length;i<j;i++){ // DECLARE VARIABLE if "i" is defined in window then it'll be overwritten
           Code in for loop should be fine 
        }

When it comes to reponseXML your XML MUST be VALID to your browser expecially. This code was tested on Chrome 22.