OS X Mountain Lion: Revert to a previous OS X version
If you use Time Machine to back up your computer, you can easily revert to a previous version of OS X if you experience trouble after installing an update.
- Print these instructions by opening the Help Viewer’s Action menu (looks like a gear) and choosing Print.
- Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys.
- 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 OS X, choose the most recent backup.
- Follow the onscreen instructions.
If you backed up any individual files as described in the Important message above, you can restore them now. If you backed them up using Time Machine, open the Time Machine menu in the menu bar, choose Enter Time Machine, and retrieve your files.