Instructions
Digital Upgrade
Navigate to the Battle.net website and log in to your Battle.net account.
Click on the "World of Warcraft" account you want to upgrade in the account list.
Click the "Digital Purchase" button.
Click "Upgrade to Wrath of the Lich King," then click "Continue."
Enter your payment details, tick the "I have read and I accept the following Terms of Sale" check box, then click "Continue." Your credit card is charged and you can immediately play "Wrath of the Lich King" content.
Purchase the Retail Version
Purchase a boxed copy of "Wrath of the Lich King" from your local game store.
Navigate to the Battle.net website, log in to your Battle.net account and click on the account you want to upgrade in the account list.
Click the "Enter Game Key" button.
Type the game key found inside the "Wrath of the Lich King" box in the text box, then click the "Upgrade Account" button.