In order to import or update Supplier information in feed you need to download the Excel template from: Integrations - Excel - Excel import: Supplier.
It is also possible to export the supplier list, make adjustments, and import it to feed.
If you are updating information you need to use the same import code that is used on that supplier. You find this information in feed under Basic data - Product: Supplier. If you enter an import code that is not in feed, it will create a new supplier.
Often supplier is created via an ERP to feed integration, but if not all fields are covered in an integration you can update them by Excel import.