Transactions cannot be cancelled directly in the app. Whether a cancellation is possible depends on how far along the transaction is.
Contact support via in-app live chat as soon as possible with your transaction ID. If your transaction has not started processing yet, the support team can cancel it for you. Once processing has started, cancellation is no longer possible.
Please Note: The sooner you contact support, the higher the chance a cancellation is possible. Delays reduce the likelihood significantly.