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
LR CC and 6 are the exact same download. It is how it is registered that make it either the CC version or the 6 version. CC is registered with an Adobe ID that has a subscription plan. 6 is registered by an Adobe ID that is not associated with any subscription plan and you then enter a serial number for it. Along with a serial number for a previous version if you buy the Upgrade version. If you buy the Full version then only the serial number for 6 is needed.
The previous posters are absolutely correct. The only thing to add is that there are some differences between LR6 vs. Lightroom CC (including the trial version) that you should probably be aware of, so that you know exactly what you're getting before making your purchase.
Enjoy the new version!