If you have access to the newer version of InDesign, this is the standard and most reliable way to "downsave" a file:
Step 2 – Go to Aconvert.com. Upload your INDD file. Convert it to IDML. Free. how to convert indesign file to older version free
For one-off conversions when you don't have InDesign access, online tools like Convert.Guru and FileProInfo provide acceptable results for simple documents. Just be mindful of privacy and file size limitations. If you have access to the newer version
Adobe’s servers will process the file in the background and send back a temporary IDML file. Adobe’s servers will process the file in the
the file. You can now open this .idml file directly in your older version of InDesign. Alternative "Free" Solutions
In the dialog window that appears, look for the dropdown menu at the bottom. Select InDesign Markup (IDML) .