If you need to delete a food or drink from your diary you can do this:
In the mobile app:
Select the food from the list in your diary. On the next screen you will see an image of the food, scroll down to the bottom and you will see a message that says "Remove This From My Diary" - push on that statement and a pop up will display asking if you are sure. Select yes and it will be removed from your diary.
On the website:
Go to My Diary by clicking on Daily planner at the top menu. From the list of foods, simply go to the bin icon and click on it. This will remove the food from the diary. Note you will not be asked to confirm if you wish to delete.