To import and only update the featured image, on the permissions step you need to set update to "only the following fields", and set the fields to:
attachments.0
0 would only import the 1st attachment row, 1 the second and so on.
If you want to import all attachments you would set it to:
attachments.*
screenshot attached of the permissions section
Oh nice. Thank you very much, it worked!
Best regards