So, updating is easy. First, check to see no database updates. Second, back up your database and files in the directories (you should already have a mechanism in place to do this, if posible via a script to automate it). Third, download and extract 4.6.4.tar.gz. Delete the favicon.ico, the htaccess file and default/settings.php. Overwrite existing files. Easy easy.

Of course if you modified your existing default theme's instead of copying it and creating it under a different name you may have some issues.