You can view and update your Coconut subscription payment method by heading to the 'More' tab in the app > tapping 'Your Plan' > 'Manage my plan' > then selecting 'Payment Methods'.
You'll be able to view your current payment method from the 'Payment methods' screen and tap 'Edit payment method' to update it. If you don't yet have a payment method set-up, you'll be taken directly to add one after tapping 'Payment methods'.
Here you can enter your new card details then tap 'Update'. For Direct Debits, you'll be prompted to 'Proceed to review' then 'Update' when you're happy with what you have entered.