You can only cancel an order if the same was placed in the advanced trading platform as "Limit Price" and while it's still not processed/fulfilled.
To cancel, access your Trade page, at the bottom center in “My orders”, you have the access to your latest placed orders.
Once they are accepted, they are not showed anymore in your orders, but in “Exchanged Trade History” at the top right, which cannot be modified/ cancelled - Advanced Trading Platform
For orders placed in the Market "at Quick Trade", they can't be cancelled.
¹ When cancelling a trade order, please notice that the remaining funds from this order will be sent back to your correspondent wallet.