Swipe through your deliveries to the order you want to change. As long as the week hasn't been skipped and our kitchen can make room for the order, you'll see 3 dots. Tap that button and you'll see a menu of options. Choose "Change number of meals" to change the plan size. Whatever plan you decide on, will only update for that specific week, rather than all future orders.
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.