How can I get rid of OS X El Capitan and go back to Mountain Lion?
How can I get rid of OS X El Capitan and go back to Mountain Lion?
OS X Mountain Lion (10.8.5)
How can I get rid of OS X El Capitan and go back to Mountain Lion?
OS X Mountain Lion (10.8.5)
How do I downgrade my Mac OS X operating system
However the real question is why do you want to downgrade, perhaps addressing the problems you are experiencing would be a wiser approach.
How can I get rid of OS X El Capitan and go back to Mountain Lion?