As of October 2008, Brightboxes are being pre-built with Ubuntu 8.04 Hardy. Previous to this, Brightboxes were Ubunutu 6.04 Dapper.
Due to the way Brightboxes are built, you can't upgrade your Dapper box to Hardy. Upgrading will break your Brightbox - it will not boot and cannot easily be made to boot.
Currently, the only supported way to upgrade is to file a support ticket with us and we'll wipe and rebuild your Brightbox as Hardy. This will require a few minutes downtime of the box and redeployment of your app (a good time to move to the new Brightbox deployment gem if you haven't already)