You can cancel your subscription at any time by going to the Subscription section found in your account Udacity Account Settings, and then click Cancel.
From there, you will be guided through some prompts in order to complete the cancellation. When your cancellation has been processed, you will receive a confirmation email.
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In addition to this, we always recommend you to cancel your automatic subscription a couple of days before your next billing date in order to avoid being billed for the next period's fees.
Post the end of your refund period, if you cancel your subscription from a Nanodegree program, single paid course, or executive program, you will continue to have full access to the classroom content until the end of the current period. There is no refund for the remaining period of the month in which you canceled.