Scroll to the bottom of your meals
Tap "Create New Meal"
This will create a new meal on this particular date. Creating one-off non-recurring meals is a Premium feature.
You can customize your daily recurring meals by editing your Daily Goal:
Tap Goals (the ⛰ icon)
Tap into your goal
Scroll down the Goal Meals section and make your edits (rename, add goals, re-order, add new meals)
Tap the ✔ in the top right corner to save your edits.