If are on a computer that doesn’t have network access or if you have are on a slow connection, it’s easier and faster to upgrade using a disc or an ISO image file already downloaded.
To upgrade from an ISO file, you need the Alternate version CD, NOT the regular Ubuntu Desktop CD. Alternate CD ISO image file of any release can be found here: http://releases.ubuntu.com/.
Once you have the image, mount the ISO image file directly on your Ubuntu system using the following command:
sudo mount -t iso9660 -o loop PATH/TO/ISO /cdrom
That will mount the ISO image to the cdrom device interface.
Once mounted, start the upgrade using this command:
sudo /cdrom/cdromupgrade
That will display a dialog prompting you to upgrade using that CD. Continue following the onscreen messages.
Upgrade via ISO progresses much like the network upgrade. Though, keep in mind that upgrading using packages on the image file only upgrades to the package versions in the official release, not the latest updated versions available on the Ubuntu repository.