if i cancel my aws exam before 24 hours will i get the refund of my voucher which i have used and can i re-use it to book a new appointment?

Hello.

If your voucher has not expired, you can reuse it by canceling your exam reservation.
https://aws.amazon.com/certification/policies/before-testing/?nc1=h_ls

Please note: You can reschedule your exam up to 24 hours before your scheduled exam time. Each exam appointment can only be rescheduled twice. If you need to reschedule your exam a third time, you will need to cancel your exam and schedule it for a more convenient date. If you paid for your exam using a voucher, verify the voucher expiration date prior to modifying your existing registration. Exam voucher expiration dates cannot be extended.