To make a payment from the app, you would click on the menu in the upper left and select Payments. You'll see this screen if you have already set up your online payment method.
In order to change the total amount that you're paying for, click on the blue checkbox next to the amount number. This will enable you to click on a pencil icon to edit the amount.
After editing the number, tap on the blue checkbox again if your "Make Payment" button isn't clickable. Now you should be able to make a payment.