You can go
here. If you are concerned about using the link I provided, you can get there yourself by going to Blizzard's WoW site, clicking on the 'Support' link at the top right corner of the page which will then take you to their support site. From there, choose the World of Warcraft option, then either the 'Billing & Account Services' or the 'Account Security and Recovery' option. Then go to any of the relevant choices, and you'll be walked through from there. Good luck!