1. Edit your /etc/apt/sources.list as root. Change every occurrence of gutsy to hardy.
   sudo gedit /etc/apt/sources.list
2. sudo apt-get update
3. sudo apt-get dist-upgrade
4. Check that Hardy was installed correctly
   sudo dpkg –configure -a
5. Reboot and Test the Upgrade
   sudo lsb_release -a

Hopefully, you should see something like

Distributor ID: Ubuntu
Description: Ubuntu 8.04
Release: 8.04
Codename: hardy