The OS X Yosemite public beta gives us some insight into Apple's next-generation operating system, but it is a beta release--a version of the Mac OS that will have problems and will not be compatible with some of your existing software. If you've leapt aboard the beta without thinking carefully about what this would mean for day-to-day use, and you now wish you had Mavericks (OS X 10.9) back, we're here to help.
Unlike with iOS--where Apple generally doesn't let you revert to previous versions of the operating system--you can reinstall a previous version of the Mac OS any time you want. But it's not as easy as you might believe. You can't, for example, simply run the Install OS X Mavericks app to install Mavericks over the top of the Yosemite beta--if you try, you'll be told that you can't install an older version of OS X over this most recent version. Similarly, if you boot your Mac into Recovery Mode (by holding down Command-R at startup) and choose to reinstall OS X, your only option is to reinstall the version of the Mac OS that the Mac is currently running (which, yes, would be Yosemite).
So, what to do? You essentially start from scratch.
Before you begin
In the hope that you're reading this prior to installing the Yosemite (OS X 10.10) beta, you should make a complete backup of your Mac running Mavericks. This ensures that if something goes wrong, or if you're simply not satisfied with the Yosemite beta, you can easily return your Mac to the state it was in before embarking on the beta. (Although, as we explained in our Yosemite beta FAQ, you really should test Yosemite on a separate Mac, or at least a different hard drive, than your "main" Mac.)
Save your Yosemite data
If you've installed Yosemite and created documents that you'd like to keep, you should back them up, as well. If you have a small number of documents you can just copy them to an external drive--a hard drive or flash drive, for example--or to a cloud service. Once you've returned to Mavericks, you'll then copy those files back to your Mac. If you've created or added more than a handful of files, you're better off backing everything up using Time Machine or some other backup solution.
Get a copy of Mavericks
In order to reinstall Mavericks, you'll need a copy of the Install OS X Mavericks app (a.k.a., the Mavericks installer). If you've heeded our warnings, you haven't installed the Yosemite beta on the only volume that can boot your Mac, so you can still boot your Mac into Mavericks from another drive or volume. Do so now and see if you still have a copy of the Mavericks installer on that Mac (it would be in the main Applications folder). If you don't, you'll need to open the Mac App Store app and download a fresh copy.
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