All credits are applied automatically to your next booking.
If you have credit on your account it will automatically be applied when you place a booking. The fixed fare displayed will be reduced by the amount of your credit. In order to redeem the credit, simply choose the 'stored card' as the payment method.
If you cancel your booking, the credit will be returned back to your account for future use, however, credit cannot be used to pay for cancellation fees, that will be charged to your credit card.
Your credit is valid for 60 days and you can view your credit amount through the promo code section of the app menu.