To add a new card, tap the 'Account' button at the top left of the app home screen or access the 'Account' page on the web. Scroll to 'Payment Cards' and tap 'Add New Card'.
Fill in the details, and select the new card as default. You can then delete your old card - just tap on it and hit 'Delete'.
If you're adding a new card while a job's in progress, don't delete the old one just yet. Instead, tap on the new card and then tap 'Make Default'. Once the job has been completed and paid for in the App, you can delete the original card.