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 Advanced Trade page, and you will have access to your latest placed orders below the Interactive Chart in "Order Management"

Once they are accepted, they will no longer show in your orders, but instead 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.