Since OS-X Mavericks (10.9) Apple have been releasing their OS X upgrades for free. This means if you have any version of OS X newer than 10.9 then you can upgrade it to the latest version for free. There are three ways you can upgrade.
- Download the Installer from Apple directly to the computer that you want to install the new OS X onto.
- Take your computer into the nearest Apple Store and they will do the upgrade for you.
- Make a USB thumb drive installer that you can plug into your computer to upgrade it.
Method 1 is Apple’s suggestion method, but if you have more than one computer it requires you to download the OS X update more than once. Method 3 gives you a thumb drive that you can use multiple times to install OSX, and you can even boot off it in an emergency if your hard drive crashes. It is very handy to have a bootable version of the latest OSX on a thumb drive lying around So I suggest option three
This article tells you how to make a USB thumb drive installer so you can upgrade any computer to the latest OS X.
It also tells you how to work out what is the newest version of OSX that you can run on your computer.