You are unable to change the details of a request once submitted or confirmed BUT you can adjust both the start and the end times when you choose to "End Sit and Pay Sitter." If you and the sitter have agreed on an earlier or later start time, you can just adjust the start time at the end of the sit before confirming payment:
Your other option is to cancel the reservation and create a new request with the correct start time. Some parents prefer to do this because we add the sit details as confirmed to the sitters phone calendar and send sitters text message reminders 24 hours and 3 hours before the start of the sit. If you are concerned about your sitter forgetting about the adjusted start time this is usually the best option.