HW 7 is the only version available.
It shouldn't matter that you don't have HW 6 installed. You can install HW 7 on a machine with HW 5 or even without Hauptwerk at all.
In any case, you'll want to preserve your settings by making a backup under the File menu of HW 5, then restore it in HW 7 later.
You buy the 6 -> 7 upgrade, get the activation code by email and redeem it in iLok, download HW 7, and install it. Details are in the first few chapters of the HW 7 Installation and User Guide, and you'll want to follow the upgrade instructions. It's not nearly as hard as going from 4 to 5.