I would suggest you to take the Airport Express to Hong Kong station, walk to "Star Ferry Pier" and take the ferry to Tsim Sha Tsui (Kowloon Side of the Habor). Maybe walk along the waterfront stop by the coffee shop/Cafe of the Cultural Center, then take a cab/shuttle bus back to the Kowloon Station of Airport Express and heading back to the Airport. There is a Restaurant for Cantonese Or Dim Sum Lunch at the cultural center but you time maybe too tight for a slow lunch. This option save transportation time and since you can (is it still availible? I don;t know) get the same day return for the price of single for the Airport Express Train.

Howwver, if you want to see the actual city more and don't mind to spend more time on bus, get a Airbus from Airport to Tsim Sha Tsui (Star Ferry). Walk the waterfront, and maybe a few block of Nathan Road, then take star ferry to Central, walk to Airport Express Hong Kong station to head back to Airport.
This way you can see more of the city, especially the Nathan Road on the way. Even better, the bus travel on the top deck of Tsing Ma bridge and give you nice view of and around it.

Give the search function a try, there are many posts about how you gonna hald a day in HKG during transit.

I really hope you can have moe time in Hong Kong to experience the city

Enjoy your trip and the stopover in HKG, home of HK-UMICH