After the 7-day refund window has passed, deferments are not an option. Please make sure you enroll for a term only if you are able to commit to the entire time frame. If you face an urgent personal issue that requires you to defer your term, please reach out to Student Services at support@udacity.com for assistance.
If you paid and enrolled for a Nanodegree program that has not started yet, you can go ahead and cancel your enrollment anytime before the program starts and re-enroll at a later time when you are ready to learn.