Once a sit is confirmed, if you need to edit the start time you can do this quickly and easily in the app.
Find the sit in your My Sits section and you'll see an "amend" option at top right of the screen.
Use that to go in and edit the start time.
Of course, we strongly advise that you double check with the babysitter first that the new start time is OK for them.
If it's not, simply cancel this sit before going on to book someone else.