It's all fun and games until someone gets hurt. Sure, installing a beta operating system can be fun and games--you get to try out the new features and see the improvements. But there is always the possibility of getting hurt, like if your software doesn't work or your Mac crashes. That's the risk of using a beta operating system.
So you installed the El Capitan beta and had your fun and games. It's time to switch back to Yosemite. But how? Booting into Recovery mode won't work, since this method can only be used for reinstalling the system that's currently on the Mac (in this case, El Capitan). You can't run a Yosemite installer within El Capitan, because the Mac always wants to have the latest version of OS X.
Reinstalling Yosemite involves removing El Capitan, doing a clean install of Yosemite, and then restoring your data from Time Machine. It's a time-consuming task, but this guide will help you get through it.
As I mentioned, you'll restore your data using Time Machine. This means you need to have a recent Time Machine backup from when you had Yosemite on your Mac. (Hopefully you read our El Capitan FAQ and heeded our call to back up.) If you installed El Capitan on a test machine and you don't need to worry about any created files, hey, that's one thing you don't need to worry about.
Create a Yosemite installer drive
Before you wipe out that El Capitan installation, create a Yosemite installation drive. There are three ways to create a Yosemite installer drive, and all of them require you to download the Yosemite installer from the App Store. It could already be in the Purchases section of the App Store app; if so, click the Download button if it's not on your Mac. If it's not in Purchases, search for Yosemite in the App Store. (After you download the installer, it will auto-launch. Just quit the installer.) You also need a Mac-formatted drive with at least 8GB of available storage.
Once you've downloaded the Yosemite installer (yes, you can do this on El Capitan, you just can't run it) and you have your drive, head on over to Dan Frakes' article on how to make a bootable OS X Yosemite install drive. The article was written from the perspective of using Mavericks to create the drive, but the steps still apply in El Capitan and Yosemite. In Dan's article, I always go with Option 1: Use createinstallmedia--it's the easiest method and it works for me every time.
Back up your El Capitan data
If you created any files while using El Capitan, now's a good time to save them. (The public beta has been out for about a week, so maybe you have just a few files.) Copy the files to an external drive (different from the Yosemite installer drive) or to a cloud storage service like Dropbox.
Say adios to El Capitan
Created a Yosemite boot drive? Check. Saved your files on your El Capitan Mac? Check. Now it's time to erase the El Capitan volume--yes, you have to erase it, you can't simply install Yosemite over it. If you try to, you'll get a message like the one in the screenshot to the above right. You're going to use Disk Utility on the Yosemite installer drive you created.
- Attach the Yosemite installer drive to your Mac, and at startup, hold down the Option key.
- You should eventually see a drive icon labeled Install OS X Yosemite. Click on the arrow below it.
- An OS X Utilities window will appear. Select Disk Utility and click Continue.
- Once the disk Utility app launches, select the Mac's hard drive in the left column (it's the first one listed).
- Click the Erase tab, and in the windows that appears, name your hard drive, select one of the OS X Extended formats.
- Click the Erase button to wipe out everything on the hard drive.
Once Disk Utility is done reformatting, quit the app. This will take you back to the OS X Utilities window. Now, select Install OS X, click Continue, and proceed through the installation process.
Restore your files
Connect up your Time Machine drive and run Migration Assistant (located in /Applications/Utilities). This will walk you through the procedure of restoring your files from a Time Machine backup. If you copied files from El Capitan to a drive or in the cloud, you can bring them into Yosemite.