Navigate to the "Billing" tab in your dashboard.
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Under "Subscriptions", you will see the "Cancel subscription" button. You can use this to downgrade to the free plan.
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You can select your reason for cancelling the subscription on the confirmation screen and press "Cancel subscription" to confirm the cancellation.
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Once you cancel your plan will not be renewed.
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Any unused credits can be used until the expiration date (1 year from the date of purchase).