The steps are explained in our FAQs.
How can I reschedule or cancel my exam?
You will need to contact Pearson VUE online at least 48 hours prior to your exam, or by phone at least 24 hours before your exam. Pearson VUE charges a rescheduling fee of U.S. $50 and a cancellation fee of U.S. $100. Please note, once you schedule your exam, any reschedules must take place within 365 days of your original appointment.
(ISC)² Community Manager