There are several ways to update Drupal. One of the easiest ways is using a patch file. There is not a lot of documentation about that, but here is how it works:

Example: Update Drupal 7.10 to 7.12 using patches

cd /yourdrupalroot
wget http://fuerstnet.de/sites/fuerstnet.de/files/patches/drupal-7.10-to-7.12.patch
patch -p1 --dry-run 

More information can also be found on http://fuerstnet.de/en/drupal-upgrade-easier.