I have been following the links provided in similar posts but it seems impossible to find where to do an upgrade from Lightroom 4 to LR 6 for Mac. I have found it earlier. Is the possibility to purchase a standalone upgrade removed. I will not go for the monthly subscription option.
Lightroom 6 is no longer avaialble to purchase from Adobe, and they no longer make the download widely available either. It might be possible for you to find some third-party outlet still selling it, or perhaps on sites such as eBay. If you are able to purchase a valid serial LR6 number you should be able to get a download link from Adobe's Customer Care team.