Apart from the obvious answers, already given (simply use Save as to downgrade your file to the required version), you could try changing the extension to .PDF and open the file from within Illustrator. With a little luck you're able to retrieve and use most of the file's content. Rather copy the content to a fresh Illustrator (CS6) file, in stead of simply re-saving the former PDF file.

Some background on this: the .AI file is since some early CS version actually non-existent. In fact, the AI file is a PDF file, with extraneous native components in it, in case Illustrator needs them to ensure edibility.

Oorspronkelijk geplaatst op Quora: How do I save an Illustrator CC file to CS6?"