By default, the drush pm-update command will update Drupal core and all enabled 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.)

It's a good idea to take a backup first:

drush archive-dump

Examples:

drush pm-update drupal-6.28

That will upgrade your site to Drupal to version 6.28, even if the latest version is 6.29 or higher.

drush pm-update drupal-8.3.0-alpha1

Update to the first alpha release of Drupal 8.3.