Telephone the company directly and ask to be put through to their reservations department. Give them your name and the date of your reservation and they will be able to cancel it for you.
Email the company if you prefer this method of communication. In the subject field of the email type "Reservation cancellation." Address the email to the "Reservations department" and make sure you state your name, the date and the details of your booking. Alternatively you could do this by handwritten letter and post it to the company, addressing it to the reservations department.
Visit the company in person if this is more convenient for you. Ask to speak to the reservations team and tell them the details of your booking and that you would like to cancel it.
Use the electronic Internet booking and cancellation system if the company has one on its website. If the company has this option you will have been given a unique booking reference number when you made the reservation. You can use this reference and your booking details to log into the booking and cancellation system on the company website. Once you are logged, you can view the details of your booking (such as arrival date and number of people) and you will be given the option to cancel your reservation with a click of a button.