Go to Tools - Advanced Settings in ITAM Data Importer. Verify Insert or Update is selected.
When you select a primary key - it will perform a lookup on the field. If the record exists, then it will perform an update on all the fields, except the key field. If the record does not exist, then it will attempt to insert a new row. You only need to use the primary lookup on enough fields to get a unique record.
Example: If all the serial numbers are unique, then use just the serial number as the primary lookup field.