Any open order may be cancelled from the Orders Pane of the Trading Page. This pane can be opened and closed by clicking on the Clock icon in the Layout Panel on the left side of the page.
The first tab of the Orders Pane is the Open Orders tab. It contains all the necessary information about all orders the current user has open. On the far right of this tab there is the option to individually cancel each order, or if you wish to cancel all currently open orders.