Hello and welcome to my User Tip.
This Apple support article explains how to restore a previous OS X version using TimeMacine, provided you made one before upgrading OS X.
(also works going back to 10.8)
Before proceeding, I very highly advise you copy your Users folders to another regular external drive (not only TimeMachine) as to be able to restore manually if you should require do so.
TimeMachine restores sometimes fails which because of unknown corruption on the boot or TM drive.
So far there has been quite a few attempts to restore a previous OS X verison via TimeMachine and they haven't been successful, thus people come here for further assistant with a manual method.
If you take the proper precautions with a extra copy of just your users files on a regular external drive (GUID/OS X Extended Jouranled formatted in Disk Utility) then you can proceed to visting the more complicated manual method below.
This is the more "manual" method to restore a previous OS X version to your machine, useful for fixing issues as well, fresh install etc.
It's a lot more work involved, but insures a cleaner install as only files are returned, not everything like TM or reverse clone does.
If you made a previous bootable clone, then hold option key down (wired or built in keyboard) boot from it, then use Disk Utility to erase the entire disk0 or drive, then use Carbon Copy Cloner to reverse clone.