Update multiple table from CSV in Cpdeigniter

[eluser]Ignis Avis[/eluser]
I am parsing a CSV file to add recrods to a table. this is how I do it.

Code:

foreach($csvData as $key => $row) {
            $data_n[$key] = array(
                'mf_date' => $row['mf_date'],
                'mf_work_id' => $row['mf_work_id'],
                'mf_sender' => $row['mf_sender'],
                'mf_amount' => $row['mf_amount'],
                'mf_trx_id' => $row['mf_trx_id'],
                );
            $this->db->insert('monthly_fee', $data_n[$key]);
        }

I have another table named monthly_due where there is three columns

Code:

1)md_work_id
2)md_ue
3)md_paid

When uploading a csv file to store data to table monthly fee
i also want to update the monthly_due table where mf_work_id from csv matches md_work_id.

Code:

md_due = md_due - 'mf_amount'
md_paid = md_due - 'mf_amount'

How should I do it?