Upgrade Mountain Lion to Yosemite

Please see How to install OS X Yosemite on your Mac - Apple Supportfor Mac System Requirements. If you are trying to get Yosemite and are using a Wireless connection, I suggest using a wired connection instead.

Upgrade from OS X v10.8 Mountain Lion to OS X Yosemite

1. If your Mac can run OS X Mountain Lion, it can run OS X Yosemite.
2. Back up your Mac.
3. Get OS X Yosemite from the Mac App Store.
4. Double-click Install OS X Yosemite to begin installation.

Reference - http://www.apple.com/support/osx/upgrade/ .

I rebooted my MacBook (2013/14) to sell, then my Wife wanted it, so I am setting it up for her. So it has Mountain Lion OS 10.8.5 installed and I want to reinstall OS X Yosemite.

A 2013/14 Mac should be running Mavericks/Yosemite when shipped. Mountain Lion makes no sense. Can you look at OS X versions and builds included with Mac computers - Apple Support and verify your model/year and the OS X version shipped with your Mac. The other option is to run Internet Recovery (Command+Opt+R - OS X: About OS X Recovery - Apple Support - with the spinning globe) and click on Reinstall OS X and check what is offered by Apple servers. The offered version is what your Mac originally shipped with.

If you do have Mac originally shipped with Mountain Lion, you can upgrade to Yosemite as mentioned previously in the steps.