Ubuntu is usually good at highlighting the existence of a new version of the distribution through its Update Manager. That means that 8.10 should be made available to you at the end of the month so long as you have sorted the relevant setting for 8.04 to realise what has happened. That lives in System > Administration > Software Sources > Updates. If you haven’t done that, then 8.04 will continue regardless since it is a long term supported release.
Otherwise, it’s over to the command line to sort you out. One of the ones below will do with the first just carrying out a check for a new stable version of Ubuntu and second going all of the way:
sudo update-manager -c
sudo update-manager -p
if you are feeling more adventurous, you can always try the development version and this checks for one of those (I successfully used this to try out the beta release of Intrepid Ibex from within a Wubi instance on my laptop):
sudo update-manager -d
Neither of the above are available in Debian so they seem to be Ubuntu enhancements. That is not to say that you cannot force the issue with Debian; it’s just that the more generic variant is used and, unless, you have gone fiddling with visudo, you will need to run this as root (it works in Ubuntu too):