You can only buy the upgrade from Adobe. Diks are provided for the full version only via re-sellers. You can buy from the following link. After clicking "Buy" change from Full to Upgrade; then select your existing LR product from the drop-down menu and the correct pricing will be displayed. Then simply choose “Add to Cart”
Or you can take a 30 day free trial via the Creative Cloud to test what’s new (some new features are only available to CC subscribers) and after the trial it’s possible to convert the CC version to the LR6 stand-alone version, upon purchase of a serial number and without any further installation. In either case make sure your computer meets the minimum system requirements.
Minimum system requirements
* GPU functionality only available on Mac OSX 10.9 or 10.10