Yes, you can upgrade for free. Instructions for doing so are here
It's almost guaranteed that you'll need to get an activation key from us, so don't worry about entering your order number and password. Just install, click "Activation Support" on the game's launcher screen, and email us the hardware fingerprint so we can send you a key. Include your order number or first/last name or the email address you used when we ordered so we can verify that you bought the game before.