You can change your payment method anytime before the payment is successfully paid to Shopee.
Payments that are yet to be completed can be found on the My Purchases page. To change your payment method go to the Me tab > To Pay > select the relevant order > Change Payment Method > select available payment method.
You should make your payment by the date found on the My Purchases page.
If your order is not found on the To Pay page, this means that your payment has been successfully processed with the previously selected method.