Users will cancel their paid Prime membership any time by visiting their Amazon Prime Account and canceling the Prime membership from there themselves. If you quit your paid Prime membership during 1st three days of signing up and paying for Prime membership, Amazon can issue a virtual e-credit note to the user then refund the full membership fee that was paid to Amazon.

How to get the refund for Amazon Prime
After canceling and quitting your Prime membership, make sure that you have also turned off the auto payments option. when doing so, you may never be charged by Amazon without your permission. Once the refund has been initiated, it will reflect into your account within 3 to four working days.

There are several cases that add to the refund eligibility. If the users have not used any of the Prime services within their paid membership, they are eligible for a complete refund. just in case they have used the services, they will get a partial refund.
https://amazonprimerefundinfo.com/how-to-get-the-refund-for-amazon-prime-refund-for-amazon-prime/