I want to downgrade from Yosemite 10.10 to Mavericks or previous operating system
If you have a recent Time Machine backup from before you upgraded, you can restore the backup and you will be back on Mavericks. You will lose any new files, changes, etc. that have been made since you upgraded to Yosemite, unless you back these up before you attempt to downgrade.
Follow these steps, from OS X Daily:


Reboot the Mac with Yosemite and hold down Command+R to enter Recovery mode (You can also hold Option and choose “Recovery HD”, or if you have a Yosemite USB installer key, you can boot from that too)

At the OS X Utilities menu, choose “Restore From Time Machine Backup”

Connect the Time Machine drive that contains the most recent Mavericks backup to the Mac (typically through USB or Thunderbolt), then click “Continue”

At the “Select a Backup Source” screen, choose the Time Machine volume used for the Mavericks backup and click Continue

Under “Backup Date &amp; Time”, select the most recent backup made from OS X Mavericks – double-check the OS X Version menu to make sure it’s “10.9.5” (or whatever 10.9.x you had previously), then click Continue again

Choose the Destination drive (the Yosemite volume you want to downgrade) to restore the OS X Mavericks backup to – this will erase OS X Yosemite and revert back to Mavericks on that drive, be sure you backed up all files created between the most recent backup date and now or you will lose them – click on “Restore” to begin the downgrade process


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The downgrade process can take quite a while depending on the size of the backup, the amount of files on the drive, the speed of the Mac, and the speed of the hard drive. Expect this process to take several hours if not many hours, for a very large hard drive of stuff, it could easily take overnight if not a full day to complete hundreds of GB of file transfers. Do not interrupt the restore process, otherwise you would need to start over.