You can select any plan or food, click "Delete" and it will purge the selection from the Meal-plan screen. However, when a day or a meal is selected, by using the Delete feature it will only purge the food content from these listings. This way, you can refill your plan without having to create a new one.
If you want to reduce or increase the number of days in a plan, click "Edit plan" and change the plan length to a new value. The program will subtract or add the number of days according to the new data from the last day of the original plan length.