Ah, one of our favourite features of Meal Garden. If you want to edit an existing recipe (to make healthy substitutions, or to put your own spin on them), go to the recipe that you want to edit and click "More Options...".
In order to edit a recipe, you have to first make a copy. Click "Make a Copy".
This will bring you to the recipes ingredient list, which you can go ahead and play around with to your liking - just remember to save your changes (and watch the health rating change - in real time!):