Hi,
After the execution of your Bracket Order(BO), you will only be able to exit the BO position. Only open unexecuted orders can be cancelled.
Lets assume you placed a Buy BO. You can quickly exit this BO position by finding the Bid Price from the Market Depth window and modifying your Sell Limit target order to a price slightly below this Best Bid Price and your BO position will be squared off.