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.
If you've ignored our advice in other articles and installed Yosemite on the only volume on your only Mac, you must depend on the generosity of your friends. That means turning to a friend (who hasn't installed the Yosemite beta), begging to use his or her Mac, and downloading a copy of Mavericks from the Mac App Store using your account.
But even here, there's a hitch: If your only Mac is a newer Mac that shipped with Mavericks, you can't download the Mavericks installer from the Mac App Store--your Mac uses OS X Recovery to reinstall OS X. And because you updated to Yosemite, recovery mode now reinstalls Yosemite.
In this case--you've installed Yosemite on your only Mac, it's a newer Mac, and you don't have a backup that contains the Mavericks installer (or is a full, bootable clone of your pre-Yosemite installation)--your only option for getting a copy of the Mavericks installer is to rely on the generosity of friends. Specifically, you'll have to beg one of those friends to lend you their copy of the Mavericks installer.
Choose (or create) an install drive
In order to use the Mavericks installer, you must boot from a volume with Mavericks (or an earlier version of OS X) already installed. As mentioned above, if you boot from a Yosemite volume and attempt to run this installer, you'll be denied. So if you have a partitioned volume that can boot into either Mavericks or Yosemite, or an external volume that boots into Mavericks, start up from that Mavericks volume.
If you don't have such a volume, but you do have a copy of the Mavericks installer app, glom onto an 8GB-or-larger flash drive and follow the advice in Dan Frakes' seminal How to make a bootable Mavericks install drive. (This is different from his just released How to make a bootable OS X 10.10 Yosemite install drive.) You'll use the techniques described in the article to create a bootable Mavericks installer volume. Of the options outlined by Dan, I've found using DiskMaker X the easiest.
Wipe the Yosemite volume
When the Yosemite beta was installed on your Mac, the startup volume was changed in significant ways--so significant that if you attempt to install Mavericks over Yosemite, you'll find that the Mac doesn't behave properly when booted into Mavericks. For this reason, you must start clean by erasing the volume. Naturally, you should back up any data on the Yosemite volume before proceeding.
If you've booted from a Mavericks volume, you can erase your Yosemite volume by launching Disk Utility (/Applications/Utilities), selecting the Yosemite volume on the left side of the resulting window, choosing the Erase tab, and clicking the Erase button. (You needn't bother with the Partition tab or Partition options, because the volume is already formatted properly if it was running Yosemite.)
If you've booted from a USB stick that you created using Dan's instructions, choose Disk Utility in the OS X Utilities screen and then erase the volume that currently holds Yosemite. Quit Disk Utility and then click Reinstall OS X. You Mac will install the copy of Mavericks that's on the USB stick.
Restore your files
Once your Mac has returned to Mavericks, restore any files you created under Yosemite. If you copied those files to a drive or cloud service, simply drag them to your Mac. If you used Time Machine under Yosemite, make sure the drive you backed up to is accessible to your Mac, launch Migration Assistant (found in /Applications/Utilities), and choose to restore your files from the Time Machine backup. If you initially backed up your Mavericks volume by cloning it to another bootable drive, simply boot from that drive, erase the volume that contains Yosemite, and then use your cloning utility to restore the clone back to the now-erased volume.