The Zsync information on https://help.ubuntu.com/community/ZsyncCdImage is showing information for the build: raring. An update to saucy would better be suited.
Updating the ISO
Ubuntu archives provide .zsync files using quite similar URIs about the various flavours. For example:
zsync http://cdimage.ubuntu.com/daily-live/current/raring-desktop-i386.iso.zsync
will sync the server's daily unstable raring (13.04) desktop image (for i386) to your local system with an older desktop image already stored on your hard drive.
Note 1: If you are using other flavours of Ubuntu don't forget to add the flavour's name in the zsync path after cdimage/, e.g.
zsync http://cdimage.ubuntu.com/kubuntu/daily-live/current/raring-desktop-i386.iso.zsync
zsync http://cdimage.ubuntu.com/lubuntu/daily-live/current/raring-desktop-i386.iso.zsync
Note 2: If you are using another architecture don't forget to change i386 in the zsync path with the correct architecture name, e.g.
zsync http://cdimage.ubuntu.com/daily-live/current/raring-desktop-amd64.iso.zsync