If you realized that you cannot make it to a shift that you have booked, you can request a swap.
View only your scheduled shifts by tapping on the "SCHEDULED" Green tab.
You can request a swap either by tapping the swap button in the shift list below the calendar, or by tapping on the shift in the list and tapping on the swap button below the shift details. You will received a popup confirming the swap request.
Upon requesting the swap, you can confirm the swap from the home screen, both in the list and in the calendar by seeing that the date has become Blue: