If you are running OS 10.9, you can go directly to OS 10.11 (El Capitan).
Open up the App Store app and download and install El Capitan.
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Personal note: Whenever I do a major system upgrade, I make a clone of my OS partition and upgrade one of the partitions. Or, I make a new blank partition and install a “clean” OS on that. (BTW: during a clean install, the installer will ask at some point whether you want to import OS settings, apps and data from another partition on your Mac. Your choice: clean install, or bring over the apps and OS settings and data you are using now.)
Why do I make a clone? Reason 1: to have a back up in case the install is interrupted by a power failure. Reason 2: Some apps or drivers may not work with the new OS. I can always go back and start up from the old OS partition if I need to use those apps or drivers.
In order to be sure that your machine supports the new MacOS and what features it does not support we have made a detailed list of all the MacOS Versions.
