Just 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 those and you'll see a few options. From there, select "Reschedule delivery" to change the delivery day - The days shown are based on kitchen availability.
Whatever day of the week you choose, only that order will be updated, not future orders. So to make permanent changes to your subscription, you'll first want to update all orders showing in your "Deliveries" tab so that your change reflects sooner.
Once that's done, tap on the person icon at the bottom right (in the Android app, you'll find this in the menu located at the top left corner). From there, tap "Manage Subscription." You're then free to choose your new delivery day by tapping "Preferred Delivery Day". This will update all future weeks that aren't currently shown under upcoming deliveries (about 4 weeks from today).