HT1443: What is an OS X reference release?Learn about What is an OS X reference release?
Currently Being ModeratedSep 22, 2013 3:21 PM (in response to IndoAmerican)
Choose About this Mac from the Apple menu and check the processor.
If it's a G3, no further upgrades are available.
If it's a G4 or G5, buy a retail Mac OS X 10.5 DVD from a source such as Amazon or eBay; Apple still might have copies in stock if you phone them.
If it's an Intel Mac, buy a Mac OS X 10.6 DVD from the online Apple Store.