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.
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.