You can cancel a Direct Debit from the app. Just go to the Scheduled tab in the Payments section, tap the payment you want to cancel, then tap the three dots (Android) or 'Manage' (iOS) in the top right and tap 'Cancel payment'. You'll need to enter your password to confirm the cancellation.

It's important to let the organisation know that you no longer need their subscription or service and that you've cancelled the Direct Debit, otherwise they may still expect your usual payment.