Can I modify an order after it's been placed?
No — once an order is placed, it can't be modified.
Can I cancel it?
If the order hasn't been processed yet, yes, you can still cancel it. Check the order's status first — if it shows as Placed, it hasn't been processed and is still cancellable.
How to cancel an order
Log in to your Customer Area.
Switch to your merchant account (if applicable).
Go to In-person payments > Orders and returns.
Check the Status of the order. If it's Placed, it hasn't been processed and can be cancelled.
Select the order and click Cancel this order.