First of all, you have to purchase the upgrade from PS CS5 to CS6.
Then you will be provided with the serial number and download of CS6, use that to install and use CS6 on your computer.
I Have purchased the upgrade from PS CS5extended to CS6. I have a serial # for CS6.
I get the message-"Serial number you provide is valid but a qualifing product could not be found on this computer"
I still have PS5 extended installed on my computer.
Is your Photoshop CS5 Extended a standalone product or part of a Creative Suite?
Where did you purchase the CS6 upgrade?
PS CS5 Extended is a standalone product.
I purchased the CS6 upgrade on line through Adobe store. Regards Mary