All rides are paid for from your account balance, which is loaded through your Balance Auto Update plan. The amount you selected when you first signed up for the app will automatically load when you start your first ride. That same amount will reload to your account balance when it reaches €0. This can happen more than once if you take many rides and/or long rides. This reload will transact automatically from your default payment method, which you can change in the Payment section of the app. If you have a new rider coupon, it will automatically apply to your ride before your account balance is used, but you will need to select a Balance Auto Update Plan prior to taking a ride.
To change your Balance Auto Update amount you simply need to toggle the switch off and then on again. You may need to have a €0 balance when you switch the toggle for the selection screen to appear.