how to travel from excelsior to HKIA early in the morning. have to catch a plane at 6.55am.

In such an early time, it is suggested to take the public bus line N11 from Paterson Street, Yee Wo Street Station to reach HKIA directly. The bus takes about 65 minutes.
This line starts from Central Bus Station at 00:20, 00:50, 01:20, 01:50, 02:20, 02:50, 03:20, 03:50, 04:20, 04:50, so you may need to take care of the time.