Can I download an older version of macOS?
Open the Mac App Store (choose Store > Sign In if you need to log in). Click Purchased. Scroll down to find the copy of OS X or macOS you want. Click Install.
Can you downgrade Mac version?
You can use internet restore to install the previous upgrade and then run the installer you downloaded. Internet restore will load the original OS that came with the Mac. You can also run the installer on another partition, if the Mac shipped with an older OS.