When a new release of Dynamics 365 is released, existing online environments are scheduled for an upgrade. If the date selected is not optimal, an administrator can reschedule that update one time.

But what if you later change your mind and want to reschedule it again? the Instance picker will not let you reschedule an update a second time. Does this mean you are stuck?

No–it simply means that you cannot reschedule it again via the O365 console. Log a support ticket with Microsoft support, and your friendly Microsoft support tech will be glad to help you reschedule the update for the desired date.