Save an InDesign CC File to CS6 (Save Backwards to an Earlier InDesign Version)

If you are working in InDesign CC and want to be able to open your document in CS6 (or earlier), you need to export an IDML file.

In the newer software, e.g. InDesign CC, export (File > Export) the document to the InDesign Markup Language (IDML):

Then open the IDML file in the older software (e.g., InDesign CS6 or CS5). It will convert it to an untitled InDesign document.  

Saving a document in a previous version of InDesign is also referred to as “saving down.” Content created using features specific to the later version of InDesign may be modified or omitted when you open the document in a previous version of InDesign.

The current IDML format replaces the Interchange (INX) format used for saving backwards in previous versions (CS4 and earlier).

To open the InDesign CS5 document in InDesign CS3, open the exported IDML file in InDesign CS4, save it and export it to InDesign CS3 Interchange (INX), and then open the exported INX file in InDesign CS3. 

You might need to make sure that the computer running the previous version of InDesign is updated with the appropriate plug-ins so that it can open the exported IDML file. The person running InDesign CS4 can obtain the compatibility plug-ins by choosing Help > Updates and following the prompts. Plug-ins can also be installed from the Adobe website: Visit Product updates page on the Adobe website, and choose to the InDesign from the product list. 

— http://help.adobe.com/en_US/indesign/cs/using/WSa285fff53dea4f8617383751001ea8cb3f-6d52a.html

Read more about saving backwards in InDesign here: http://forums.adobe.com/docs/DOC-1024