You can now buy account-upgrade keys and download the game client for World of Warcraft: Wrath of the Lich King
directly through the account-management section of www.worldofwarcraft.com. The direct online upgrade method is
the most convenient way to upgrade to Wrath of the Lich King and join the millions of players who are already
adventuring in Northrend.

All you need to do is:

That’s it! If you haven’t upgraded your account yet, now’s your chance to take your World of Warcraft experience to the next level, whenever you want.