Are you talking about CSV import?
If so, then you can both add new records by importing and update the existing ones. If you do not specify an ID of the record in the CSV file Aware IM will consider it a new record, otherwise it will use the record identified by the ID (by the way, a record can also be identified by a combination of attributes, not just by ID - you need to place a "*" symbol next to the name of the attribute in the header of the CSV to indicate that the attribute(s) identifies an existing record.