how to update a xml file using Dom object on button click

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XML

Hi I am creating Xml string from an Xml file like this

xml=loadXMLDoc("data.xml");
parser=new DOMParser();
xmlDoc=parser.parseFromString(xml,"text/xml");
x=xmlDoc.getElementsByTagName("A")[0].childNodes[0];
x.nodeValue="123";

I am confused how to use the xmlDoc object to update the data.xml file on button click.