How to downsave an InDesign file to open it in an older version
Follow these simple steps in order to open an InDesign file in an older version of the app (CS6) which was created in a newer version:
- Open the file in the newer version.
- Select Save As or Save a Copy (in case you need to keep a newer version as well).
- Choose IDML file format (compatible with CS4 or later) and save.

Note – Packaging the InDesign file also include an IDML version of the file.
Check this help article for more details: https://helpx.adobe.com/in/indesign/using/saving-documents.html#save_backwards_to_the_previous_indesign_version
