Upgrading a Drupal site refers to moving it’s database and files from one major version of Drupal to a later one, to take advantage of the newer features and bug fixes that might not be available for the older version. Upgrading a Drupal site is a pretty effective way to make sure that your Drupal site is currently up to date with all the latest security updates and bug fixes. Old versions of Drupal will naturally be unsupported with the arrival of the new releases. The Drupal community has currently stopped supporting the older versions of Drupal like version 4 and 5 and has moved to Drupal 7. However Drupal 6 is more widely supported. Have a look at the steps required to upgrade your Drupal site from an older version to Drupal 6.