Swipe through your deliveries to the order you want to change and you'll see three dots in the top right-hand corner. Tap them, then "Change number of meals" to change the plan size for one week.
To make a more permanent change, tap "Account" and then "Manage subscription" - In the Android app, you'll find "Account" in the menu at the top left corner. From here you can select a new meal plan size for all future orders, updating all weeks that haven't currently been charged.