Open gnome terminal and type below text
user@user-desktop:~$ sudo mount -o loop ~/Desktop/ubuntu-12.04-alternate-amd64.iso /media/cdrom0
If the upgrade dialog is not displayed for any reason, you may also run the following command using Alt+F2:
gksu "sh /cdrom/cdromupgrade"
Or in Kubuntu run the following command using Alt+F2:
kdesudo "sh /cdrom/cdromupgrade"