You can unreserve the vehicle by going to the vehicle's Reserve Vehicle section, then simply cancel the reservation.
After this first step is complete and the vehicle is no longer reserved, if it has been removed from your Sales Channels make sure to go back to Business -> Sales Channels to re-publish it onto your website and third-party sales channels of your choice.
If a sales invoice has been created that will need to be cancelled before the reservation can be cancelled, for guidance please refer to our Cancelling A Vehicle Sale or Reservation guide.