Sometimes you need to download an older archive version of Drupal. This is often the case when you are upgrading an older version of Drupal to the latest version. You would then download the older archived version of Drupal in order to do a "code diff" with current site to determine which site files have been "customized" or changed. That way, you got a list of file changes that you might need to "code merge" the changes to upgraded version.

To download older archived versions of Drupal go to Drupal.org and then go to "Download -> Drupal Project -> Find Out More -> View All Releases". Then find your release and look for the link besides "Download:".