To change a specific food in the Meal Plan, simply click the food. You'll see a list of options with similar nutritional values. Choose the option you want by clicking the Choose button and it will automatically be substituted into your Meal Plan.
To change an entire meal, click on the "Substitute" button to the right of that meal. At this point, you can substitute individual items still (with the Substitute button), or choose another meal from the list (with the Choose This Meal button). When you choose a meal, it will automatically be substituted into your Meal Plan.
To add foods not in the substitution list, click “Add a Food” and search for other foods to add to your tracker for the day.