Upgrading from 1.3.2 to 1.5.4

[eluser]Unknown[/eluser]
I have inherited this system. It works fine, but I would like to update the code base to the newer framework. The user guide suggests making incremental updates from each version. However, there doesn't seem to be any download for previous versions. Can somebody provide the archive location for old versions, or give the instructions for a jump greater than one minor version?

Thanks,
Chris