From my experience you can reschedule the exam as many times as you want, as long as you do so at least 48 hours before your current appointment. I rescheduled my exam at least 4 times before I finally went for it.
You can reschedule the exam online or by calling the test center directly. But note that many test centers do not provide direct (phone or otherwise) rescheduling facility. For such centers, you can reschedule the exam online.
Check out the Prometric's FAQs page for more details.
Note: It is best to call Prometric and confirm the terms & conditions of rescheduling the exam in order to avoid any issues.