Go to the Streams View by selecting the 'Streams' button at the top-right of the the PureCM Web Client. You can drill down into the stream folders by selecting the name until you find the stream you are looking for.
The button to the right of the stream with the tooltip 'Show History' will show a list of the submitted changesets for this stream.
Select the 'Ref No' of a changeset to view the changeset details. Within here you can select a path to view the file differences.