There are two ways:
If you want to extend a confirmed booking by a few days, you can make a new Direct Booking for the additional dates with the same sitter.
If you want to pick your dog up later or drop them off earlier, you can discuss this with the sitter through Booking & Messages (or the Inbox in the app). They they can send you a supplement to the booking, which you can easily pay online. A supplement can be for any price you've agreed on.