Ha! I didn't find the link until I had already downloaded, installed and paid for my first version of Lion. After I found I had been charged, I asked for a refund and that's when they told me to go to the update link! I had to go through all that including downloading Lion again with my redemption code and re-installing it. After I did that and provided the redemption code, they agreed to issue me a credit for the original purchase of Lion.
Why didn't some kind of instructions come with my newly purchased MBP? I bought it on 28 June, well after the announcement that it would be free. Would have saved me about half a days work!