How do I travel from HKIA to Excelsior Hotel? By bus or train? Thank you...

Either way can be ok:
By bus: take shuttle bus no. A11 and get off at Sugar Street, Yee Wo Street. Then your hotel is around 250yd away. The fare is 40HKD and the travel time is around 70min.

By train: take Airport Express train to HK Station and transfer to free shuttle bus line H2. It directly takes you to the hotel. In this way, the cost is 100HKD and it takes around 45min.