Execute Script - How to Update second node in XML group

Hi,

I have an XML variable "myXML" which is set to:

<root>

     <nodes>

          <node>

               <value>a</value>

          </node>

          <node>

               <value>b</value>

          </node>

     </nodes>

</root>

Question - within an Execute script, how can I easily access and update the second node value? i.e. change "b" to "c"

The following fails in Execute Script (although it is possible using an XPath expression in a SetValue

  patExecContext.setProcessDataValue("/process_data/myXML/root/nodes/node[1]/value", "c");

If I have square brackets in this expression then it fails with a nasty error

Is there an easy way?