Forgive me if I am missing the obvious. I picked up a base model Macbook Air on Best Buy's sale but decided I want to exchange it for a 256ssd model. When I got it, I installed Yosemite beta. Tonight, I went through the process to erase and reinstall OS X via internet in order to get it back to Mavericks. The process went through and after about an hour, the OS downloaded it and was on the new Mac startup screen. I have always heard that this process reinstalls the version of OS X that the computer shipped with (which would be Mavericks) but to my surprise, it downloaded and installed Yosemite. Any direction would be greatly appreciated. I am trying to get it back to factory Mavericks so that I can exchange for the 256SSD model.