Full answer
The exact path: log into Amazon.com, hover over "Account & Lists" in the top right → click "Prime Membership" → on the Prime page click "Manage membership" on the right → click "End membership and benefits." Amazon will walk you through a confirmation flow with several screens designed to make you reconsider.
Refund eligibility depends on benefit usage. Full refund: if you haven't used any Prime benefits (shipping, video, music, etc.) since your last renewal charge. Partial refund: prorated based on how much of the membership period is remaining and which benefits you've used. No refund: if you've heavily used benefits in the current billing period.
If you're still in a free trial, cancel before the trial end date to avoid any charge. Amazon emails a reminder 3 days before the trial converts to paid. If you miss it and get charged, you have 48 hours to cancel and request a refund if no paid benefits were used.
Instead of canceling, consider pausing. Amazon allows you to pause Prime for up to 3 months through the same "Manage membership" page. During the pause, you keep your data and cart but lose access to benefits. Good option for people who travel or have seasonal usage patterns.
After cancellation, you keep Prime benefits until the end of your current billing period. Download or watchlist any Prime Video content you want to save access to — content rented through Prime Video (not bought) will no longer be accessible after the period ends. Same for Prime Reading borrows and other library features.