Yes it is possible, but why would you want to?
6.whatever is not stable yet. Do not take this the wrong way, but if you are not sure how to upgrade to what will probably become 6.04 you probably cannot handle the problems that will come with running a testing distro.
If you want the most current Ubuntu, that is 5.10. You can use apt to upgrade. Simply edit /etc/sources.list to replace every instance of "hoary" with "breezy" then save it.
Then, open a command prompt and type
sudo apt-get update
and
sudo apt-get upgrade
Good Luck!
I've been wondering if it's possible to install Ubuntu 5.04 release, then use apt to upgrade to 6.? release. Can anyone tell me if that's possible and how to use it?

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