From the "My Entries" page you can edit the account items for any entry that hasn't been submitted yet. Just click the button to "View/Edit" and you will be referred to a page where you can change the quantity of any items requested. To get rid of an item entirely, just set the quantity to blank or 0.
Then click "Submit Changes to Your Account" and you will be referred back to the "My Entries" page where you can see you balance has updated.
*We've edit the entry blank to make it clearer that you need to enter a quantity, not the price.