This update is only for MkIV nav systems, and reportedly it's just a maintenance release with no new features. It will take your system to 4-1/50.
The V25 image files, zipped and in CloneCD format, can be downloaded at this link -
http://www.megaupload.com/?d=02GKUQO5
Be sure to use quality CD-R media and burn at the lowest possible speed. Also note that there are risks to doing the upgrade on your own, so do so knowing that risk is present. Although rare, problems can occur if update steps aren't followed properly or there are issues with the media. Such conditions can cause problems with the nav drive which may not be recoverable.