If you go to the Slysoft website, you'll see a page for purchasing their products: http://www.slysoft.com/en/purchase.html
There is a small box there that says Upgrade AnyDVD to AnyDVD HD. It also says: If you already own a standard AnyDVD license, you may upgrade here to AnyDVD HD. To upgrade, you will need your current AnyDVD serial number and the email address you used when you ordered your standard AnyDVD license.
Just click on the Buy Now button to start the process.