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”
If you don't have a previous version of LR select "Full"
Or you can take a 30 day free trial via the Creative Cloud and convert the CC version to LR6 upon purchase and without any further installation.