To delete an item in the Diary, go to the meal, click on More Information. Then simply swipe over the food item from the left to the right. You can also delete the food from the meal list (as well as edit it (weight/number of portions/etc.) by tapping on the item to see the detailed view. In order to delete an item tap on Delete.
Simply hold the finger one second on the item in the Diary you want to delete, and when the confirmation pop-up appears, tap on OK to confirm the deletion. You can also tap once on the tracked item in the Diary and then tap on the bin icon in the right upper corner to delete the item from the list. In the same view you can edit the tracked food, simply save the changes by tapping on the green button showing the tick to the right.