1. You need a Holdem Manager Account in which your license key is registered. If you don't have one yet you can create one for free here and register your license to it by clicking the "Upload License Code" link in My Account view:

2. Depending on what type of license you possess, you are granted a rebate towards an upgrade. In this case I get $75 off for possessing a pro combo license (plus i have $5 remaining get it free credit from an earlier transaction). I conduct the upgrade by clicking the "HM2" Button to get to the product page and chosing the desired upgrade product from the "Buy Now"-dropdown:

Details on the upgrade policy and how much credit is granted for what license type can be found here: HM2/OM2 Price & Upgrade Policy

3. Now I have this upgrade poduct in my cart and can either proceed to checkout or continue shopping and go back here later by clicking the according link in the top right corner of the page. Checkout process should be self-explanatory of you ever shopped online.

4. Tadaaa: I have been issued a regular HEM2 pro combo license:

This is also visible in "My Account" view:

5. Don't forget to enter your User ID under the following link to get your 500 Bonus SPs:

Buy Hold'em Manager 2 or Omaha Manager 2