Here's what I need to to to update drupal (minor updates, e.g. from 7.18 to 7.19) on my Ubuntu server. I have installed drupal from the drupal source itself and not from the Ubuntu repository (because the Ubuntu repository is usually quite old and not updated as frequently). Actually the update is quite painless; I am sure somebody automated that already somewhere...
tar -xvzf drupal-7.19.tar.gz
chown -R jeltsch:www-data drupal-7.19