Contact the Help Desk or your scheduler. They are usually accommodating. I am presuming you are referring to the Blue Portal.
I had one shop which was inaccessible due to construction and they rescheduled it. But normally, there is an area where you can actually reschedule it yourself. If not, then contact the Help Desk and explained your situation.
If you are referring to Purple Portal, I can't help you because a simple question could result in something else with the Help Desk or whatever.