Replace data instead of new entry

Hi everybody

I need to replace data of firms in Microsoft CRM 2013 (Service Pack 1 (SP1)(6.1.4.212) (DB 6.1.4.212)). If I download an update the firmdata and try to import it, it generates a new entry altough the firm-id and the firm-name are identical. How can I make sure that it replaces the data instead of generate a new entry?
This is a one-time import. I don't need to do this on a regular base.

Best regards

Valentin