If you are unable to go on your holiday, you can transfer your booking to another person who meets the terms and conditions of the package. You must notify the tour operator or travel agency of this well in advance of the holiday. You can be charged a reasonable fee for this, up to the actual cost incurred by the organiser to transfer your package travel contract.
Your right to cancel your booking depends on the terms and conditions of the tour operator or travel agency in question. There are differences between tour operators and travel agencies, and you should always read the terms and conditions carefully if there is a chance that you might not be able to go on the holiday.