If you send a booking request and change your mind before the landlord accepts, you are free to withdraw it.
To withdraw a booking request, go to your Personal Dashboard by clicking on your name on the top right-hand side of your screen and click on "Booking requests". Find the booking request you wish to withdraw and click on "Withdraw request".
Your booking request will be automatically withdrawn and you won't have to pay anything for it.
This doesn't apply to Instant Bookings, as you are accepted immediately to the place.