***I am not a teacher***
"Should I update the Excel sheet as per the client's letter?"
and this one, I would probably write as follows (assuming you address your boss as 'Sir'):2) I need to send mail to my boss regarding my one of team member that he is not in office today.
"Hello sir , Pattric is not in office today because his brother is admitted in hospital at morning. and Pattric need to reach there for take care his brother."
"Dear Sir, Pattric is not in the office today as his brother was admitted to hospital this morning, and Pattric needs to visit the hospital to take care of him."
Hope this helps.