How do I downgrade from Yosemite 10.10 to Mavericks 10.9?

Backup Mavericks using Time Machine first (if possible)

Follow these steps to get a copy of Mac OS X Mavericks:

1. Open the App Store.
2. Enter your Apple ID and Password and click Sign In (if App Store requests it).
3. Click Purchases.
4. Scroll down to find Mavericks.
5. Click Download.

Now that you have downloaded the Mavericks file you will find it inside Applications. Revert to Mavericks by creating a Mac OS X USB boot drive.

How to make a bootable Mavericks install drive

You can use this file to turn an 8GB USB Flash into a Mac OS X Mavericks boot drive. With this USB Boot Drive you can wipe the hard drive on your Mac, and install a fresh copy of Mac OS X Mavericks. If you have your Mavericks Time Machine backup, it can be used to reinstall all your files and programs (otherwise you’ll need to setup your Mac from scratch using the files you backed up to an external drive or cloud storage solution).

Using a Mac OS X Mavericks boot drive to perform a fresh Mac OS X installation

1. Shut down your Mac (Choose Apple > Shut Down).
2. Insert the Mavericks USB Flash Drive into your Mac.
3. Startup the Mac while holding down the Alt key.
4. Choose the USB Flash Drive Volume.
5. Choose Utilities > Disk Utility on the Install Mac OS X window.
6. Select your main Volume in the sidebar. Now click Erase > Erase (warning, this removes everything on the volume).
7. Click Disk Utility > Quit Disk Utility.
8. Tap Continue on the Install OS X window.

Check out this MacWorld article. It explains each step in detail.

Note: In some rare instances, it may be necessary to format the Macintosh HD partition before installing Mavericks. **This will erase your entire hard drive ** so be sure to back up any files you need before going this route. I've outlined the steps to do this below:

1.) Boot the Mac from the USB you made using the steps above. 2.) Open Disk Utility and erase the drive entitled (Usually) "Macintosh HD". You can rename it whatever you like. 3.) Install Mavericks.

If you had a Time Machine backup you can revert pretty easily: Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts. Select “Restore from a Time Machine Backup,” and then click Continue. Select your Time Machine backup disk. Select the Time Machine backup you want to restore. To restore your computer to the state it was in before you installed the new version of Mac OS X, choose the most recent backup. Follow the onscreen instructions.

If you didn't you're in for a much tougher time. I'd recommend you visit the Genius Bar at the Apple Store. But basically, you will have to erase your entire hard drive and re-install the OS X version of your choice.