If a sitter needs to add a day to the beginning of a request that has already been accepted, do they have to cancel that request and send a new one or can an existing booking be edited?

A much easier way would be to have the person book the first day as another stay. It has the benefit of you getting paid for that day before the rest, plus it will give you a repeat customer!

If you need to change an upcoming or pending stay just choose the client, then if you are on your PC -in the details box on the left there is a modify booking "button", click it change what you need to and "book it". The client then needs to accept the changes to finalize. On your cellphone - choose the client's stay, click the details bar at the top, then "modify" in the top right corner, then choose modify details. Finish the same as on your PC.