Though an order has already been placed, you could still cancel it.
Click on the menu icon on the top left hand corner of the app and then click on "Track Order".
Once the order page opens, you can click on "Cancel Order" to cancel it.
Please keep in mind that cancellation from app is possible only until the order is scheduled for pick up from the seller. However, if the product is already with us, cancellation would not be possible from the app.