We use your bank account number and sort code as the destination to send your funds to at the start of the loan. These details are not used for payments as we process payments via CPA from your debit card.
As a result, you're unable to amend your bank account number and sort code during the course of a loan.
Instead, you will need to wait until your current loan has been fully repaid before changing your card details. You can amend where you'd like the funds to be sent to, here.