can I upgrade from mac os x 10.6 to Mavericks ?

If you have previously downloaded Mavericks, and if your computer is compatible: see http://support.apple.com/kb/HT5842

Otherwise, Mavericks is no longer available for download.

Only if you downloaded the full installer for it before Yosemite came out and your Mac meets the system requirements for it; if so, log into the Mac App Store with the Apple ID you used and click on the Purchases tab.

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