zsync allows 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.
Install zsync, open a terminal and run the command:
apt-get install zsync
Or download the source tarball and compile:
wget http://zsync.moria.org.uk/download/zsync-0.6.2.tar.bz2 tar -xjvf zsync-0.6.2.tar.bz2 cd zsync-0.6.2/ ./configure make sudo make install
Open a terminal and navigate to the folder that has the .iso file you want to update.
Find the http:// path of the corresponding zsync file on Ubuntu's website here to the .iso you want to update.
Run the command substituting file names:
zsync -i ubuntu-your-version.iso http://releases.ubuntu.com/karmic/ubuntu-your-version.iso.zsync
zsync -i ubuntu-10.10-desktop-amd64.iso http://releases.ubuntu.com/10.10/ubuntu-10.10-desktop-amd64.iso.zsync
Watch the magic happen, remember to delete (or rename) the original .iso file and rename the updated .iso file.