Zsync

zsync allow you to update an existing ISO file to match a later release, for instance if you have downloaded a beta version of Ubuntu and you want to update it to the current release. To be able to do this there must be .zsync file available, in the following example the shows updating direct from Ubuntu's website. You could download the .zsync file and do things locally.

First install zsync, open a terminal and run the command:

apt-get install zsync

Next navigate to the folder that has the ISO file you want to update, run the command

zsync -i ubuntu-9.10-beta-netbook-remix-i386.iso http://releases.ubuntu.com/karmic/ubuntu-9.10-netbook-remix-i386.iso.zsync

Watch the magic happen, and remember to rename your updated ISO file