To change the payment method of a registered payment, navigate to Financial Management → Order Management.
From the next page, click the icon Manage installments and payments (2nd icon) on the line of the order concerned
At this point, at the bottom of the screen, you will see a list of payments received, then recorded, related to that order; you will then now be able to correct the date of receipt and method of payment.
Click the green button Update Payment to save the transaction.
On the other hand, if an invoice has already been issued for a payment, it will no longer be possible to change the payment method from the payment itself, but it will be possible to change it from the invoice (click here for the specific guide)
If, on the other hand, you use single billing for multiple payments (i.e., single billing for all installments of an order), you will also be able to make changes from the payment itself, thus from order management.