I have an iMac that has not been used in 3 years and had OS X 10.5.8 on it. I ran software update and am told the software is up to date. I would like to upgrade to Mountain Lion but the Apple support seems to say I need to have 10.7 installed already to do that. How do I get past the 10.5 that I have now?
You need to call Apple: 800-MY-APPLE and purchase Snow Leopard on DVD.
Once you install Snow Leopard, run Software Update again, which will update it to 10.6.8. Then, you can buy Mountain Lion from the App Store.
Apple support seems to say I need to have 10.7 installed already to do that.
They're wrong. You can skip Lion.