I want to upload a csv file that contains data the updates both the primary table and the secondary table. How would I do it, can I simply reference all the fields that will be updated? What happens if the primary table already has data (the user date is primary table). The secondary table is the amount and dates of payments with some other data attached.
Importing data to normal fields and to subtables would be two different step, regarding importing, please refer to this article : Importing & Exporting
General fields data should be imported first and then importing subtable data to existing entries, and you could choose the import policy to handle duplicated keys during your import.