Yes. Go to the Manage Positions page found under AutoTrading menu on left hand margin, seen here:
For each open position, you have the ability to do the following Actions:
You will see a "signal being processed" message and then you will see the new signal in black text below the original signal which is green text.
You can Add Stop Loss or modify it by clicking on Change your current stop price, or you can click Cancel to remove it altogether.
When you modify an open position, that particular position will remain untouched by AutoTrading in order to respect your modification, and will close only when the system itself goes flat that same position.
NOTE: Only market orders are supported when changing the quantity of a position from the Manage Positions page. You can also add exit stops/limits manually to close (or partially close) a position, but you can only use market orders to increase its quantity.