How can I downgrade my MacOS
How can I downgrade my MacOS
iMac Line (2012 and Later)
You have to completely erase the drive.
Start by downloading Yosemite and create a bootable installer.
Boot from the bootable installer.
In Disk Utility, select "Show all devices" from the View popup menu.
Select the drive and erase using Mac OS Extended (Journaled).
Back out of disk utility and run the installer for Yosemite.
Migrate from your Time Machine backup when the Startup Assistant asks what you want to do.