How do I upgrade drush from 3 to the latest release? On my debian 6 server, I used apt-get to install drush.  However, it looks like it downloaded drush version 3.  I want to upgrade to the latest stable release of drush (which I believe is drush 5).  How can I do this?
The short answer is on the Drush project page under the heading Install/Update - Unix/OSX:

pear upgrade drush/drush


I first had to install PEAR, but that should not be an issue.