How can I update the OS system on an iMac that has OSX Yosemite?
My 2008iMac was updated to OSX Yosemite . How can I update the OS?
iMac Line (2012 and Later)
An iMac from 2008 is too old to upgrade past El Capitan.
How to upgrade to OS X El Capitan - Apple Support
The next releases—Sierra and then High Sierra—require an iMac from 2009.
How to upgrade to macOS Sierra - Apple Support
How to upgrade to macOS High Sierra - Apple Support
A newer or new Mac is your path.
This ~12 year old Mac has served well over its years.