You can add and remove items from your order before checking out your basket. Once the order is placed, there is no option to do this.
To add an item to your basket, simply tap on the desired item, choose your options, add a note for the kitchen and indicate the number of items you would like by clicking the + or - signs. Click Add to basket when you are done.
To edit or remove the item from your order, click the View basket option at the bottom of the screen. In the basket view, you can either tap the Edit button or swipe left over the item you want to edit or remove. Select the pencil icon to edit the item or the bin icon to delete it.