This works:

http://www-128.ibm.com/developerworks/library/x-domjava/listing3.html

public static Node setValue(Node startNode, String value, String xql)
    throws Exception
  {
    Node targetNode = XPathAPI.selectSingleNode( startNode,xql );

    NodeList children = targetNode.getChildNodes();
    int index = 0;
    int length = children.getLength();

    // Remove all of the current contents
    for(index = 0; index < length; index++) {
      targetNode.removeChild( children.item( index ) );
    }

    // Add in the new value
    Document doc = startNode.getOwnerDocument();
    targetNode.appendChild( doc.createTextNode(value) );

    return targetNode;
  }