i bought a refurbished dell computer 6 years ago came with all photoshop c6 ver13 want to update

Unless you and the former owner did a license transfer, you never owned the program... which does not really matter... as stated, there is no upgrade

Transfer (selling or giving your software away) https://helpx.adobe.com/x-productkb/policy-pricing/transfer-product-license.html
-you may NOT sell or transfer an Education serial number

No need to worry about the registration details, all special deals ended years ago. Photoshop CC from $10 per month, rather good compared to the $700 for Photoshop CS6.