How to update XML using XPath and Java

You can delete the file and create a new one.

Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(
    new InputSource("data.xml"));

XPath xpath = XPathFactory.newInstance().newXPath();
NodeList nodes = (NodeList) xpath.evaluate("//employee/name[text()='old']", doc,
    XPathConstants.NODESET);

for (int idx = 0; idx < nodes.getLength(); idx++) {
  nodes.item(idx).setTextContent("new value");
}

Transformer xformer = TransformerFactory.newInstance().newTransformer();
xformer.transform(new DOMSource(doc), new StreamResult(new File("data_new.xml")));

Use setNodeValue. First, get a NodeList, for example:

myNodeList = (NodeList) xpath.compile("//MyXPath/text()")
           .evaluate(myXmlDoc, XPathConstants.NODESET);

Then set the value of e.g. the first node:

myNodeList.item(0).setNodeValue("Hi mom!");

More examples e.g. here.

As mentioned in two other answers here, as well as in your previous question: technically, XPath is not a way to "update" an XML document, but only to locate nodes within an XML document. But I presume the above is what you want.

EDIT: Responding to your comment... Are you asking how to write your DOM to an XML file after you've finished editing the DOM? If so, here are two examples of how to do it:

http://www.java2s.com/Code/Java/XML/WriteDOMout.htm

http://download.oracle.com/javaee/1.4/tutorial/doc/JAXPXSLT4.html