Do I need to upgrade my database?

After having made a backup of your old site and uploading the new files, visit http://example.com/update.php, read and act upon the instructions, then click through to "run the database upgrade script". You may expand the "Select version" fieldset to review the updates Drupal automatically selected or simply click update.

Drupal 5 and later will also alert you to run update.php if it is detected that your database is no longer up to date. The alert will appear on the Administration page (admin) and the status report (admin/logs/status).