You can go onto site and reschedule as stated above. Know however, that not attending your schedule start date appointment might lead to a significant delay in your hiring now. They tend to schedule a number of new hires for a single orientation; it is a long and detailed "event" lasting several hours. If there are no additional hires due for orientation at this time, you will have to wait until there are. I have heard of new hires having to wait a number of months for the next orientation day if they miss the one that they are first scheduled for.
You will have to go to the portal and request a reschedule
You're answer is absolutely correct regarding a possible delay of several weeks before another orientation. Nonetheless, please contact HR immediately for further instructions.
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