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.
If you want to create a new recurring meals which automatically appear on all days moving forward, tap Goals (the 🎯 icon) > tap into your goal > scroll down the Goal Meals section and make your edits. Tap the ✔️ in the top right corner to save your edits.