The simple rule is, the sooner the better.
You can cancel rental car and hotel bookings (if cancellable) yourself via the booking platform and, if necessary, rebook for the changed travel date. How do I book a hotel? How do I book a rental car?
For flight bookings, the process is a bit more complex.
If you made a mistake when booking, contact the service team immediately. With a bit of luck, the ticket has not yet been issued and cancellation is possible. Once the ticket has been issued, one can only hope for the goodwill of the airline. The best chances are here on the same day the ticket is issued.
If the flight ticket is to be rebooked, it is also best to do this early on, provided that the new travel dates are fixed. The more seats are available on the plane, the cheaper the rebooking is. If the booking is only changed shortly before departure, the surcharge may be very large.
In general, it is advisable to change your booking at least two hours before departure, as technical problems can make the process more difficult or even require you to contact the airline.