You can cancel an outstanding order using the Order Load panel on the Market Maker window.

To cancel an order from the Market Maker window, perform the following:
On the Marker Maker window, view the symbol associated with the orders you want to cancel.
Click the CXL
(Symbol) button. The Confirm Cancel
window appears.

Click the Yes
button to cancel all live and pending orders for the symbol.
NOTE: If you do not wish to automatically cancel orders when
you click the CXL button and not view the cancel order confirmation window
in the future, click the check box so that it is clear.
The order's row changes color
(the default color is red) and its status becomes Pending
Cancel.
If the cancel is accepted by the exchange, any remaining unfilled shares are canceled.
If the cancel is not accepted by the exchange, the order remains live or pending.
TIP: You can also add a speed key to quickly cancel an order.