During today's keynote,
Users of Snow Leopard (10.6), Lion (10.7), and Mountain Lion (10.8) can get Mavericks for free, regardless of whether you paid to upgrade before or if you're still using the software that came with your computer.
We've put together a quick guide to installing Mavericks that works for anyone with an iTunes account (it's required to use the Mac App Store). If you're on a notebook, make sure your computer is plugged in before you begin this process.
To get the upgrade, click this link to open the Mavericks preview page in your web browser.
Once that page is open, click the link to "View in Mac App Store":
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That'll open the Mac App Store app and take you to the Mavericks upgrade page. Click "Free Upgrade":
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The button should turn green and say "Install." Click it and enter your iTunes credentials.
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The installation will begin. A small progress bar for the download will appear beneath the Launchpad icon in your Dock:
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From there, it's a smooth process. Your Mac will ask you to restart when the download is completed. Once that takes place, you don't have to do anything but log back in to your Mac when it's done.