how to import only a section of Word doc

I can't really think of a way to make this work except to cut out the part of the Word doc you want to replace.

1. Extract the new Chapter 6 text.
2. Save it as a file of its own.
3. Place it in the ID document, perhaps in a text frame of its own.
4. Edit and format as needed.
5. Cut/paste over the prior Chapter 6.

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A couple of suggestions (possibly for the future...):

• Import each chapter separately. This might mean breaking the Word file down into separate chapters, but it is worth it in the long run. Then you only have to deal with one chapter's changes.
• Word has a Compare Document feature so you can see where the changes have been made from the previous Word file. Then you can just copy-paste the changed sections.
• Look into Em Software WordsFlow--it can link the Word file to InDesign so the author/editor can edit in Word and the changes will appear in InDesign.

if I copy and paste the changes, the formatting is lost

That doesn't have to be the case. Go to Preferences > Clipboard Handling and tick "Text Only".

And...you're using paragraph styles, right?