If you need to edit a food you can:
In the App:
Select the food you wish to edit and on the next screen you will be able to edit the serving size and or the quantity. Push on update and this will update the entry in your diary.
On the Website:
On the food you wish to edit, look for the pencil icon at the end of the line. It is after the Carbs value for the food. Click here and you will be able to edit the serving size accordingly. Click on the pencil icon again to save the changes.