I've checked the official website and seems all I can do is buy CC which I definitely do NOT want or buy a full new version of the standalone Lightroom, no upgrade option is obvious?
It is available as stand-alone upgrade Lightroom 6 (compared Lightroom CC 2015). Look in the Adobe store of your country.
Right at the bottom of the Adobe home page (in fact on almost all pages), click on the "Products" link. Then you should get the list of products, including Lightroom, which are available for stand-alone purchase. Click on "Buy", then you can change "Full" to "Upgrade".