Updating os x
Look up your service provider’s settings Unfortunately Mail may be configured to change connection settings dynamically, which in addition to being the root cause of some Mail problems, may makes the current settings in Mail unreliable to use as a reference for what ought to be correct for your account.
I know that it is possible to restore a computer to the OS it had prior to an upgrade, but is it possible to upgrade to a (never-seen before) OS version that is not the latest available?
The App Store, of course, only offers an upgrade to El Capitan, currently the newest OS X version.
Edit: This question is related (but only if Yosemite had been downloaded via the App Store in the past).
Apple has really made the Operating System interesting but many people including me are not able to upgrade the Operating System in MAC Book having inadequate knowledge about the system requirements.