I can't see anywhere on the Adobe sight to do this?
Yes, you still can. It's not the easiest to find but it is there.
Go to the Adobe main page (by clicking on the Adobe logo top left).
Scroll right down. you'll see an option for Products..
Click on Products. Scroll down to Lightroom. Click on "Buy". Change the entry to Full if it's not an Upgrade you're after, then click on add to cart and follow the on screen instructions.
Retrieving data ...