If the professional hasn't accepted your request yet:
Go to the section "Services" - "Upcoming"
Select the service
Click the "Edit date/time" button
If the professional has already accepted your request and the service has been correctly booked, you can manage it directly through the chat by asking the professional to send you a proposal with the new date or time for the service.
Once the proposal has been sent, you have 15 minutes to accept it. As soon as it is accepted, the previous service will be automatically cancelled and the new service will be booked with the updated schedule.