You can change your card details directly in the app. Just click on the 'My TruPlan' to open the payment page and click on 'Payment method'. Click on the plus sign on the top right corner and enter your new card details.
You can also use the camera function, this will take a picture of your card and add your details automatically without you having to type in your details. Please be aware that you still need to type in your CVV and postcode before the app allows you to save new details. You can enter this functionality by clicking on the 'camera' icon on bottom left.