Purchasing an upgrade is nearly the same as purchasing a regular license. Just visits this page:
and click on the button to order the license of your choice (Professional or Unlimited).
On the next screen you will need to enter your previous Order ID. This will be listed on the email you received after your initial purchase. If you can't find it then just email me at firstname.lastname@example.org and I'll look it up for you. Remember to include the name or email address you purchased under so I can locate your order.