Hi,
Before issuing a show cause letter, certain procedures need to be followed. First, you should ensure whether the updating of work schedules is mentioned anywhere, either in the job description or in emails. If yes, and still, the employee is not updating, then proper counseling should be conducted, and it should be documented. Even if the employee continues to not follow the updates, seek an explanation, and if the behavior persists, issue warning letters and finally, resort to issuing a show cause letter.