Update XML document using Java

On the internet I am finding a lot of examples of how to achieve this functionality, however none of them seem to work in my case. I have an string containing XML data, and I know what the XPath of the result should look like. Say for example I have an xml structure looking like this:

   <?xml version="1.0"?>
     <m:parent xmlns:m="http://www.somescheme">
      <doc:header xmlns:doc="http://www.somescheme">
        <doc:documentId>137</doc:documentId>
        <doc:documentDescription>Some Description</doc:documentDescription>  
         <doc:task>
           <doc:id>49</doc:id>
           <doc:name>Some Name</doc:name>
           <doc:description>Some Task Description</doc:description>
           <doc:outcome/>
           <doc:dueDate/>
           <doc:priority>50</doc:priority>
          </doc:task>
        </doc:header>
        <m:otherinfo>234324</m:otherInfo>
      </m:parent>

Say for example I need to access and update otherInfo the XPath would be 'm:parent/m:otherInfo' which I should be able to read and set. For doc:outcome it would be 'm:parent/doc:header/doc:task/doc:outcome' This is a value I need to set at runtime.

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

    XPath xpath = XPathFactory.newInstance().newXPath();
    NodeList nodes = (NodeList) xpath.evaluate("//m:parent/doc:header/doc:task/doc:outcome", doc,
        XPathConstants.NODESET);

The code should be something like above, however I am not getting results,

Can you point out what I am missing ?

Kind Regards Max