If you've submitted an order outside of market hours and subsequently had a change of heart, you do have the option to cancel it.
To cancel such an order, follow these steps:
Go to the 'Investments' tab.
Scroll down and tap on one of your Stocks.
Within the 'Activity' tab, you should be able to view your pending buy or sell orders.
Select the specific buy or sell order you wish to cancel.
At the bottom of the screen, you'll find the 'Cancel investment' button. Use this to cancel the order.
The cancellation will take effect immediately, and Plum will no longer proceed with submitting the buy or sell order on your behalf.