How to change the value of XML node using java?

I had to make some assumptions and changes because I don't know what your XML document looks like. But, the Element class extends Node and the setTextContent is a method on Node. What you need to update is the first child node, which is typically the text value of the element, but you should add some validation to make sure. Then, once you've updated the text value, you need to serialise the DOM back to whatever form it came from:

public static String changeProductVersion(String reqXML, String xpathExpression) {

    Document document = null;
    String updatedXML = null;

    try {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();
        InputSource is = new InputSource();
        is.setCharacterStream(new StringReader(reqXML));
        document = builder.parse(is);

        XPath xPath = XPathFactory.newInstance().newXPath();
        Element element = (Element) xPath
                .compile(xpathExpression)
                .evaluate(document, XPathConstants.NODE);

        if(element != null) {
            NodeList childNodes = element.getChildNodes();

            // Get the first node which should be the text value.
            // Add some validation to make sure Node == Node.TEXT_NODE.
            Node node = (Node) childNodes.item(0);
            node.setTextContent("17.1.0");
        }

        System.out.println("Updated element value: " + element.getTextContent());

        // Serialise the updated DOM
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();
        DOMSource source = new DOMSource(document);
        StreamResult result = new StreamResult(new StringWriter());
        transformer.transform(source, result);

        updatedXML = result.getWriter().toString();

    } catch (Exception ex) {
        ex.printStackTrace();
    }

    System.out.println(updatedXML);
    return updatedXML;
}