What is this rubber circle on duct
It's a plug for a hole. It should pry out - doubtful that it will allow you to adjust airflow.

Beware of random self-adjustments to a furnace's airflow done without checking the current to the fan motor - you can overload the fan (and the overload will happen in the opposite direction than you expect, unless you are unusually well educated in the ways of centrifugal fans.)