Yes. You can update to 8.10, and from that to 9.04. You cannot upgrade directly from 8.04 to 9.04. There are two ways to do this. The first, and easiest, way is to simply run the Synaptic Package Manager. It should have a small popup telling you that a new distribution release has been made, and asking if you would like to upgrade to that. Select yes and proceed.
To upgrade manually, or on a command-line system, open up a terminal and enter
gksudo gedit /etc/apt/sources.list
in a graphical system, or
sudo nano /etc/apt/sources.list
if you are using Ubuntu Server. Replace all occurences of "hardy" with "intrepid". Then run
sudo apt-get update sudo apt-get dist-upgrade
This will upgrade the system to the 8.10 release. To upgrade from that to the 9.04 release, replace "intrepid" with "jaunty".