well, some times we have earlier versions of ubuntu installed and want to upgrade.  After downloading iso, many a times we don't want to burn it to CD-ROM or to USB for various reasons. Here is a tip to upgrade directly from iso.

Copy iso to local folder. Mount the iso with following command:

sudo mount -o loop -t iso9660 /path//ubuntu-xxx.iso  /mount_point (for eg., /media/ubuntu)

This should mount iso in /media/ubuntu as in the above example. Once mounted, add following line to /etc/apt/sources.list (comment out other repositories if you want to upgrade only from iso)

deb file:/media/ubuntu/ubuntu distroname main restricted (distroname = feistry,. gutsy, maverick etc)

Now update the packages with sudo apt-get update command. Run sudo apt-get dist-upgrade. That is it.