Most Mac users are happy with OS X El Capitan, but for some situations, the new version of OS X 10.11 is unusable for one reason or another. Perhaps it’s running worse than before, slow, or unstable, or maybe some important piece of software is not compatible with El Capitan, like some versions of Office. Whatever the reason, a solution for these situations can be to downgrade from OS X El Capitan and revert back to the prior version of OS X that was running on that Mac.
You can use this approach to downgrade to OS X Mavericks, OS X Yosemite, Mountain Lion, or Lion, directly from OS X El Capitan, assuming you have a backup from one of those versions. This relies on having a recent Time Machine backup made of OS X prior to the Mac updating to OS X El Capitan. Without a Time Machine backup to revert to, this particular approach won’t work.
Before getting started: You should finish a new backup before starting this process. You should also manually copy any new files or important data or documents that have been created from the date of the last backup from the prior OS X version and now, since you will lose those files in this downgrading process. Essentially what you are doing with this method is restoring from a prior Time Machine backup of a prior OS X install.
How to Downgrade OS X El Capitan Back to OS X Mavericks, Yosemite, or Mountain Lion with Time MachineConnect the Time Machine drive to the Mac that contains the backup of a prior OS X installationReboot the Mac and after you hear the start chime start holding down Command+R to boot into Recovery Mode (you can also hold down the Option key and boot from a El Capitan installer drive)When you see the “OS X Utilities” menu on screen, choose “Restore from Time Machine backup”Select the Time Machine drive from the “Select Backup Source” screenAt the “Select a Backup” screen, choose the backup you wish to restore to, focus on the ‘Backup Date & Time” and “OS X Version” listings to make sure you are selecting the proper backup, keeping in mind the following:“10.10.5” or any “10.10.x” would be Yosemite“10.9.5” or “10.9.x” would be Mavericks“10.8.x” would be Mountain LionWhen you have selected the backup you wish to revert to, click on “Continue”Now choose the destination drive to recover to, usually this is “Macintosh HD”, then click on the “Restore” button and let this complete – this will downgrade from OS X El Capitan to whatever version of OS X and the associated backup you choseOnce you’ve started the restore and downgrade process, you should be ready to wait for several hours, depending on the size of the backup being restored, the speed of the disk, and the speed of the Mac. Downgrading can take quite a while, so be sure the Mac is plugged in to a power source and don’t interfere with the process.
After the downgrade from OS X El Capitan is complete, the Mac will restart and boot back to where it was before at the date you selected, including whatever the OS X version was at that time. So if you were running OS X Mavericks prior to installing OS X El Capitan, and you chose that date and OS, the Mac would reboot to OS X Mavericks. The same applies to revert to OS X Yosemite, Lion, or OS X Mountain Lion from OS X El Capitan.
Once the downgrade and reversion to a prior release has complete, you can manually copy over any of the changed or new files that you made earlier, otherwise just be on your way. If you plan on avoiding OS X El Capitan, you may wish to hide the update from the App Store.
Another option is to perform a fresh install of the version of Mac OS you wish to run on the Mac. This will erase everything, however, and you’d be on your own to manually backup and restore your files. If that’s the way you want to go, you can clean install OS X Mavericks, Yosemite, or, if you are troubleshooting and that is the primary reason for wanting to start over, perhaps consider staying with OS X 10.11 but performing a clean install of OS X El Capitan.
Please let me know if there is anything else I can do to assist you on this. I will be happy to continue working with you on it.