You can find a free update for Mac OS X Lion 10.7 in the Mac App Store. As of June 2015, upgrading the operating system on a Mac to OS X Yosemite is free for OS X Lion users.
Apple's website offers a complete explanation of the hardware and software that is compatible with the free upgrade to OS X Yosemite. The company's website also explains new features in the updated version of OS X, including design changes, iOS integrations and new functions such as the notification center. Users who do not have access to a broadband connection to download the upgrade from the App Store can upgrade their Macs at any retail Apple Store.