Older document versions and revisions are exported using the Log Viewer application.

Start in the Navigator. Select and right-click on the required document.
Choose the Document Revision History option.

In the Log Viewer window, Holding the Ctrl key, select the two revisions that you want to compare. Right-click and choose the Difference option.

To compare a revision with your current working copy select only one revision in the Log Viewer.  Right-click and choose the Difference option. These selected revision will be compared with your working copy.