Go to the bottom of most of the Adobe website pages and look for a "Products" link. Click that and then look for Photoshop Lightroom and click the BUY button. Then click the LR 6 BUY now button from the screen that comes up. On the screen that comes up select Upgrade and your older version of LR. The price should change from the $149 to $79.
Thanks, I'll give it a go.