Here are simple steps on how to dist-upgrade a Debian system using an ISO file.
Normally the process would be quite simple by replacing the entries in the repository list
and call a command apt-get update; apt-get disp-upgrade.
BUT what if your system doesn’t have Internet acccess while you want to do the upgrade?
Here is a suggestion on how to do it with an installation iso file.
Copy the iso file into the Debian system you want to upgrade
eg. in /data/debian/
Comment out all entries in /etc/apt/sources.list and run the following commands:
mount -o loop -t iso9660 /data/debian/debian-6.0.4-amd64-DVD-1.iso /cdrom
apt-cdrom -m -d=/cdrom/ add
apt-get update
apt-get dist-upgrade