Just drain it and refill. You have no way of accurately topping it off now.
I used Mobil 1 synthetic ATF in mine but they require a Mercon compatible automatic transmission fluid.
They were over filled at the factory which causes "windage" problems and makes them hard to shift.
The correct amount of fluid to refill them with is 3.2 quarts.
And spending the extra money on synthetic ATF is definitely worth it...