For that, you will need an ISO of older version which is readily available on any torrent sites. But even after that while installing you get errors like: This copy of OS cannot be verified or is corrupt etc. You get this error 99% of the time as Apple don’t verify older OS and wants you to install the latest one.
There is a work around which is quite simple:
Open System Preferences > Date & Time uncheck “Set date and time automatically” and set date to 2/1/2016. And then try installing the OS it will work.