You can delete individual foods from your daily log in of two ways:
Swipe the food to the left
Tap the Food to enter its Details page > Scroll down to the bottom of the page > tap the big red Delete button
Tap the ... button next to the meal name
Tap "Delete Foods from Meal"
This will delete all foods from this meal.
Tap the ... button in the top right corner of the daily log
Tap "Delete Today's Foods"
This will delete all foods in all meals from this date.