This is interesting. If you have a machine on 10.9 I'd download the installer there and use Apple's instructions (
found here) to install it on a thumb drive.
To me, it sounds like you installed either DP1 or PB1 and never updated. When I went to update from PB6 to PR I just had to click continue to download the file.
If neither one of those works, I'd backup via Time Machine, boot into Internet Recovery (if your Mac shipped with Lion or newer) by holding Command+R at start up. Erase and format your hard drive and reinstall whatever OS it prompts you to (it could be anything from 10.7-10.10 depending on your model). Once you get into the OS, download Yosemite. Note this will take a while to boot over the internet and reinstall over the internet. If you have a another Mac with 10.9 or a 10.9 USB drive, I'd go that route.