How do I download the latest operating system?
How do I download os x yosemite?
iMac, OS X Mavericks (10.9.4)
Click the App Store icon in the Dock.
After that has loaded, click the big "OS X Yosemite" banner near the top of the window. On the new page, immediately under the circle-X in the upper left will be a download button. Click it. Follow the prompts.
It is a free download.