You can buy an upgrade to CS6 but you will be two versions out of date.
IMO, you’re throwing good money after bad. There’s not all the much in CS6 than CS5.5
You can still purchase CS6 as a perpetual license, but it will not be getting any feature updates (and it's not 2 versions out of date). A creative Cloud subscription allows you to use CC2014, CC and CS6 all for the same subscription price.