No problem. To do so, You would need to cancel the order you've just placed by following the instructions HERE. Separately, if you wish to order something else OR the same product in a different size, you would need to place a new order for that product in your preferred size on the app.
In case you receive the order and have a change of mind, you may choose to opt for a different color/product OR the same product in a different size. To do so, do follow the steps HERE to return/exchange your product. NOTE: Opt to 'Return' for a different color/product and Opt to 'Exchange' to receive a different size.