If you already own CS6 (and have a valid serial number) and want to re-download it, download the trial version from
http://prodesigntools.com/adobe-cs6-direct-download-links.html
You must follow the Very Important Instructions on that page before you click the download links otherwise the links will not work.
If you need to purchase a CS6 upgrade version (to upgrade your CS5) then go to
Click the Buy button, "I want to buy: Upgrade", "I own" Photoshop CS5" and complete your purchase.