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Go to https://www.dropbox.com on your computer. Click your profile photo. Click Settings on the menu. Click the Plan tab. Scroll down and click Cancel Plan. Scroll down and click I still want to downgrade. Select a cancellation reason. Scroll down and click I still want to downgrade.

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If you're not already signed in to the account you want to delete, follow the on-screen instructions to sign in now.  Use this method to cancel your subscription to Dropbox Plus or Professional account if you subscribed on the web. This will downgrade your account to the Basic level without deleting your files. If canceling puts your Basic account over the storage quota, your automatic syncs will be paused until you delete excess files manually. It's at the top-right corner of the page.  It's near the top-center part of the page. Your subscription details appear here. You'll be asked if you want to revert back to a free account.  This information helps Dropbox improve their services in the future. Dropbox will now send a confirmation message to your email address to confirm the cancellation.  Your subscription will remain active until the final date of the current billing cycle. At that point, your account will be downgraded to Dropbox Basic. If you want to delete your entire Dropbox account, see the Deleting Your Account method.