Setyl relies on the unique identifier that is shown against each Asset downloaded from Setyl, the field is called URL. So long as you include the URL when uploading the modified data, Setyl will update all fields associated with the URL.
New Assets will be created for any records that are uploaded without a recognised URL.
How can I use the Bulk Upload feature to bulk update Assets?
What is the difference between creating multiples of one Asset and a bulk upload?