If you have an earlier version, you may be able to buy and install Snow Leopard, which will then allow you to download Yosemite you'll first need to make sure your hardware will support Yosemite. OS X Yosemite is free to download and install if you have already have Snow Leopard, Lion, Mountain Lion, or Mavericks. If you need to run PC and Mac software at the same time, you will need to buy additional software such as Parallels or VMware. The main disadvantage of Boot Camp is that you have to restart your computer whenever you want to switch between OS X and Windows. You can then install any PC software you need. Boot Camp comes pre-installed in OS X, and it allows you to install Windows on your Mac (if you have the Windows installation discs). If you absolutely need to use PC software and there isn't a Mac version, you can use a feature called Boot Camp. Click the Apple icon in the top-left corner of the screen, then select About This Mac.On a Mac, all of your system information is visible from the About This Mac window. Then you can compare it with the requirements above to decide whether you'll be able to upgrade. To upgrade to Yosemite, you'll need to know what type of hardware your Mac has, as well as which version of OS X you are using.