Have you tried reading the tutorial yet?

http://java.sun.com/xml/tutorial_intro.html

Reading is relatively straight-forward, and so is updating.
You create a Document object:

DocumentBuilder docBuild =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document doc = docBuild.parse(fileName);

...and then access / modify the nodes.

But to save your file, you need to create a Transformer object. Here is an
example (assuming doc is your xml Document):

try {
    if (doc.getDocumentURI() == null)
        throw new IOException("No file name specified.");
    TransformerFactory tFactory = TransformerFactory.newInstance();
    Transformer transformer = tFactory.newTransformer();
    transformer.setOutputProperty(OutputKeys.INDENT, "yes");
    transformer.setOutputProperty(OutputKeys.METHOD, "xml");
    transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
    DOMSource source = new DOMSource(doc);
    StreamResult result = new StreamResult(doc.getDocumentURI());
    System.out.println("Saving XML file: " + doc.getDocumentURI());
    // this line actually does the save
    transformer.transform(source, result);
} catch (TransformerConfigurationException ex) {
    throw new IOException(ex.getMessage());
} catch (TransformerException ex) {
    throw new IOException(ex.getMessage());
}

Like Christophe wrote, this is explained in the tutorial. You will want to
look at the DOM model and not SAX to see how to implement this.