You can reschedule an upcoming appointment yourself via your account; your appointments are all listed in the "Your Account" section on desktop or under "My Appointments" on the app. To reschedule, select the upcoming appointment and click the "Reschedule" button. Choose your preferred date and time and you're all set. You can also cancel the appointment from the same screen.
Note: If the upcoming appointment is less than 24 hours away, you will not be able to reschedule the appointment yourself and will need to contact your pro directly. Just click the pro's name at the top of the appointment info screen - their contact info is on their profile.