I have got a Wix Install file.

<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
  <?define ProductName="My product for ArcGIS 10.2"?>
  <?define ProductVersion="1.3.0.0" ?>
</Wix>

Where I would like to match the define with the ProductName and update the attribute value for that one. How should I select?
I tried with:

Wix/?define/@productname

and

Wix/processing-instruction(define)/@productname

But both fail in the update process.