By default, the
drush pm-update command will update Drupal core and all enabled to projects to the latest recommended releases. The optional
project argument allows you to provide a list of installed projects to update, thereby limiting the scope of the update.
You can also pass a specific version of Drupal core as a project argument and Drush will limit the upgrade to that version of core (and core only, unless you pass other projects along as arguments.)
drush pm-update projects drupal-6.28