Called Sage and found out this doesn't exist, which I am surprised to hear.

Say you need to update a customer's payment terms (after invoices already imported on existing terms).
or the payment terms were incorrectly submitted to begin with.
or the Sage payment term options are not the ones needed. (Such as EOM which would alter on relevant month the actual last day of that said month! 28th/29th /30th/31st! [Another suggestion to made]).

To update these you need to go one by one in the transaction screen and update them to the desired date.

It would be beneficial to be able to update these en masse with the import function as you can with the Customer Records details.

A static and unique reference, such as the transaction number, could be used to ensure you are not importing further data/ duplicating transactions.

It would mean a new option in the mapping section to be added if it was based on transaction number.