It is serial-number or subscription. The subscription version is easy to find. The serial-number version is available if you go to adobe.com and scroll down then click the Products link in the footer of each page then look for LR then click Buy and pick Upgrade if that’s what you’re wanting.
Keep in mind that LR CC gets feature updates going along while LR 6 won’t get any and you’d have to wait until LR 7. Dehaze seems to be the popular one LR CC has, already, that LR 6 doesn’t and we’re just up to LR 6.1.1 so I’m sure there’ll be more featuers.