Please note if you are upgrading minor version of drupal-7.21 to drupal-7.22. Then please use
1. If you are upgrading from drupal 6 to drupal 7, you must be on the latest version of drupal 6 release. 2. There is a difference between update and upgrade. 3. You should have a separate directory(target) which will be the act as active drupal root directory.
Main work starts here
Site-upgrade always upgrades from the current Drupal site to an empty Drupal site. You need a site alias to show the target website.
How to create an alias:
Create here file named as test.aliases.drushrc.php put following code in it.
$aliases['test'] = array ( 'root' => '/path/to/upgraded/drupalroot', 'uri' => 'http://localhost/drupal7', );
drush site-upgrade @test
That all folks !!!