First, install all available updates in the previous version of InDesign that will be used to open the IDML or INX file:
In InDesign, choose Help > Updates and follow the prompts.
Then choose the procedure below that corresponds to your current version of InDesign and the previous version that you will use to open the files.
When you open a document in a previous version of InDesign, you may lose content or the layout may not match the original. Newer features are often not supported in older versions. Carefully review documents opened in previous versions.
You cannot save back directly from InDesign CS5 or later to CS3 or earlier. InDesign CS5 doesn't support INX export. Save back to CS3 via CS4, which supports both INX and IDML.