you can't cancel the USMLE
the test fee ~600$ is not refundable
you can only take the test, or extend the period(for a fee)
if you dont take the test or extend, it will expire - then you have to register again = ~ $600
you can cancel or reschedule the test appointment date, within the 3month period, with prometric. (if you reschedule too late (~5days) they may charge a fee).
DO NOT SCHEDULE THE TEST (appointment date with prometric) AND NOT GO.
i think it shows up on the transcript as an attempt.