You can cancel your own order while it is still in processing and has not shipped.
Open the order from Buyer → Orders.
Choose Cancel order.
Select a reason, add any useful comment, and confirm.
Before the seller posts it
The item goes back on sale and a refund request is placed into the admin refund workflow. Your order page shows what happens next.
After the seller posts it
Once the parcel is with the courier it cannot simply be called back. This becomes a return or refund case instead. Read the refund and return article, which covers what to do.
Act before shipping
The cancel action disappears once the order has shipped. If it is no longer available, use the return option when one is open or raise a ticket from the order page.
An order that is already cancelled or refunded cannot be cancelled again. If the status looks wrong, raise a ticket on that order rather than placing a fresh order.
Whoever cancels has to give a reason, and it is recorded on the order. Say clearly why you are asking, because that reason is what support reads later if there is any argument about it.