First off, the easiest way to go about checking out Hardy is by downloading a live cd image and doing a fresh install on a separate partition. This avoids spoiling your Gutsy installation. You can do that by going here:
But if living on the edge is more your style, you can upgrade your current Gutsy release to Hardy Alpha 4 by doing the following things. Before you do so, you should know that Hardy Alpha 4 is by no means stable, and should only be done on a system that you want to play around with. That being said, here’s how it’s done!
EDITING THE REPOSITORIES
Open up your terminal of choice. First off, backup your current repository list. Type:
Now, open up the original sources.list file:
In Gedit, click Search >> Replace.

Fill in the fields as above, replacing the word Gutsy with Hardy. Click Replace All, and save the file.
UPDATING REPOSITORIES WITH APT-GET
Now type:
This will update the repositories. You may notice the Ubuntu update manager icon telling you there are several hundred updates available. Ignore it, as we’re going to update things using the terminal instead.
FINALLY: UPGRADE TO HARDY
Here’s the big step. If you’re having second thoughts, you can still back out now! Otherwise, type:
Everything is downloaded and installed. This could take a while. Go watch something on TV.
When it’s finished, you’ll need to reboot the system. Type:
And you’re finished! Hopefully it all went smoothly. It is an alpha release, but I personally haven’t experienced any major problems.
Alternative Method
There is another method which is a little easier, but also less customisable, and will only work so long as Hardy is listed as “developmental” (which means upon Hardy’s release, this method of upgrading will not work). Nevertheless, here’s how it’s done. Simply type in a terminal:
sudo update-manager -d
Release 8.04 (Hardy) will be detected and can be upgraded to by clicking the “Upgrade” button.