Once you have placed a buy or sell order while the stock market is closed, you will have the option to cancel it before the order is executed. You cannot cancel or reverse an order that has already been executed in the market.
There are two ways to cancel a pending market order:
Go to "Pending orders" or your transaction history, both at the end of the Portfolio screen
Click the pending buy or sale order
Click "Cancel order"
Then just confirm that you want to cancel the order! Keep in mind that you can only do this when the market is closed.
Securities trading is offered through Northbound Securities LLC, an affiliate of Northbound Group LLC, d/b/a Sproutfi