Happened to me too. I just kept getting a little pop up window with a message that I needed to enter the serial number of the product I was upgrading. Where?
What I finally figured out was that you have to click on the v5 product link in your PreSonus account. There's an Upgrade button on that page. Click on that, and you'll be able to enter an upgrade serial number. Convoluted.
Note to European members...
Best Service may have a slightly lower price (139 €), but many people were reporting that B.S. were unable to provide serial numbers (may have run out or may have had an IT glitch). The issue may have been fixed by now. I purchased the upgrade through Thomann. Slightly more expensive (145 €, including 20% VAT), but I already had an account and have purchased numerous items from them with no problems. They issued me a serial number immediately. It took me longer to register it with PreSonus than it did to buy it.