If your luggage will be automated transfered, you just have to show valid passport and flight ticket when go out of the airport. Otherwise, you have to collect luggages and store them firtsly.

Then, follow the directions to the airport ground transportation center to take bus A21 and get off at the New World Center of Tsim Sha Tsui. From here, you could walk to visit hte Avenue of Stars, Hong Kong Space Museum and then the Harbor City both for lunch and shopping. In the afternoon, take the StarFerry beside the Harbor City to Central and then walk to the Peak Tram Station along Garden Road. Highly suggested that you take the Tram to visit the Victoria Peak and the Madame Tussauds on it.

The time should be around 18:00 by now. You could have a dinner in Central and have a look of the featured building here. Then, you'd better walk to the Hong Kong station from Central before 20:00 and watch the grand A Symphony of Lights for 15 minutes along the seashore. Finally, you could take the Airport Express train from Hong Kong station back to the airport in 30 minutes, when it shoud be 21:00.
Ps: it will be helpful for you to learn about the mentioned Hong Kong sightseeings(https://www.travelchinaguide.com/attraction/hongkong/) in advance.