To change or update your default payment method for all current and future orders please sign into your Sezzle dashboard.
At the top of your screen select Settings > Payment Methods.
Then click on the payment method you would like to use and it will expand, you will see an option to "Set as Default".
If you need to add a new payment method, you can click on the "Add new debit/credit card" button or the "Link a bank" button.
Once the new payment method is added you can then designate it as the default by selecting "Set as Default".