The short answer: to downgrade an InDesign file like you would with a JPEG or PDF. Adobe removed this feature years ago for stability reasons. However, there are professional workarounds.
Let’s address the specific keyword phrase: how to convert indesign file to older version link
Alternatively, you can use the "Export" feature to convert your InDesign file to an older version. Here's how: The short answer: to downgrade an InDesign file