Rolling back to an earlier OS on MAC is a real pita. Basically you’ll need a copy of the installer and then you’ve got to partition the HD, install the old OS, move all of your stuff across and then boot from the new partition (old OS) and delete the partition with the newer OS on it.
Unless of course you’ve got a Time Machine backup.
You*do* have a TM backup, don’t you...?
I didn’t; that’s how I know how to do this! 😉