aptitude

As root:
aptitude
Find drush under PHP.

At the bottom of the information about the drush package, there's an i that means installed next to 2.0 and a p that means no packages or config files or anything are present next to drush 3.3.

Type + when the drush 3.3 line is highlighted.

Then type g to upgrade (twice, first it takes us to a confirmation page).

Note: On Agaric's servers the package list is updated daily by a cron job, but updating the package list can be invoked by hitting u if necessary.