OS X Yosemite: Revert to a previous OS X version
If you use Time Machine to back up your Mac, you can easily revert to a previous version of OS X if you experience trouble after installing an update.
Print these instructions.
Click the Share button in the Help window, then choose Print.
Choose Apple menu > Restart. After your Mac restarts and you hear the startup chime, hold down the Command (⌘) and R keys.
When the Apple logo appears, you can release the keys.
Select “Restore from a Time Machine Backup,” 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 OS X, choose the most recent backup that occurred prior to installing OS X Yosemite.
Follow the onscreen instructions.
If you backed up files as described in the Important message above, you can restore them now. If you backed them up using Time Machine, click the Time Machine icon in the menu bar, choose Enter Time Machine, then retrieve your files.