The most recent update for OS X Mountain Lion is 10.8.2. I have no idea what you are referring to a "mac 10.8.5" as that is neither a valid OS X release number (yet) nor does it seem to be a firmware number. Your profile says you have an iMac running 10.5.8??
Exactly what model of Apple computer are you using, and what version of OS X is it running?
If you are running an iMac older than a mid-2007 model it cannot run OS X 10.8 (see http://support.apple.com/kb/HT5444).
You'll need a lot more detail on which model iMac you have and its specifications, but that's all outlined in the link below:
Kappy 08/2012 post on upgrading to Snow Leopard, then Lion or Mountain Lion - https://discussions.apple.com/message/19401628 - including how to get Snow Leopard and Lion since Apple removed them from the online store.