Make sure you are ready to downgrade your operating system by checking in with what version of OS X you already have and ensuring you have enough hard drive room to download the older version. Clicking the apple in the top left corner of your desktop and selecting About This Mac will allow you to see what OS X version you are currently running. Edit 2016: This is true for any OS except El Capitan - Apple are leaving it available so devices ineligible for Sierra can still obtain it. You need to already have it in your Purchase History, otherwise once it's gone from the App Store, that's it, it's gone. There is no 'legitimate' way to download macOS from anything other than a Mac. It requires the Apple App store. Note: You can not use a Time Machine backup to revert to an OS which is older than the OS of the latest backup on that Time Machine drive. Installing Adobe Reader is a two-step process. First you download the installation package, and then you install Adobe Reader from that package file. If you're running Mac OS X 10.9 or later, install the latest version of Reader. For step-by-step instructions, see Install Adobe Acrobat Reader DC on. New versions of MacOS installers are typically available through the Mac App Store, accessed through a computer that is compatible with the latest Mac system software release. Older versions of Mac OS X installers can often be downloaded as well, but from different sources.
Choose About this Mac from the Apple menu and check the processor as well as the OS version.
If it's a Core Duo Mac running 10.5.8 or earlier, clickhere, install the DVD, and run Software Update. A Core Duo Mac can't be upgraded past 10.6.8.
Old Mac Os
If it's a Core 2 Duo or better Mac, upgrade it as if it was a Core Duo Mac, and once done, if desired, open the Mac App Store and try downloading Yosemite. If you get told it's incompatible, go to the online Apple Store and order a download code for Lion 10.7.