Adobe has done a very god job at hiding the link to purchase the standalone/upgrade version of the new Lightroom 6 - you can purchase/upgrade here (in the dropdown menu you can select "full" or "upgrade"). If this doesn't work, you can try this link. The upgrade price from LR5 is $79:
Adobe Lightroom 6 / Lightroom CC will be officially released shortly - I think the official announcement will be at noon EST. The official Adobe website is not yet updated, but the new LR6 is already listed at Adorama for $149.
A quick update: the new version of LR will be called Lightroom CC and I am now 100% that it will be announced tomorrow morning (EST). Some of the new features are HDR, fusion panoramas, face recognition and CC integration.