Save file as IDML from indesign cc, then open in indesign cs6
This is only an option if you actually have access to CC. I ended up installing CC on my machine along with CS6 when we started receiving CC files from our customers. Until I did that I had to call clients and ask them to resave their files to IDML and resend them.
As far as I know there is no way to upgrade CS6 to a point that you can open CC files with it.