Term-based Nanodegree Programs:
"...you hereby acknowledge and agree that Udacity will not offer refunds on any fees and charges related to your purchase of any products or Services. This includes any partially used or unused periods for which you have already paid. We do not guarantee refunds for lack of usage or dissatisfaction."
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.
Subscription Nanodegree Programs:
Please see this article for relevant information.