Please note if you are upgrading minor version of drupal-7.21 to drupal-7.21.
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.