To edit or cancel an open order, go to the open orders page.
Canceling An Order
Once on the open orders page, simply click the "x" on the rightmost column of the order you wish to cancel.
Editing An Order
On the same page as above, click the pencil button on the rightmost column corresponding to the order you wish to edit. You will be taken to a page where you will be asked to fill in any changes you wish to make. When you are done, click "Edit Order" to confirm your changes.
Changing the price of an order or setting it at market price may cause one or more trades to be made.