What does overfilled transmission fluid to a car
The fluid will start to foam and maybe blow out some of the seals due to pressure. Drain some if you think you overfilled it.
Overfilled tranny....not good. It can spray out the fill tube and catch fire on the exhaust header. Very thin oil is flammable. Hook up a thin plastic tubing and vacuum pump some out and then refill cautiously to the correct level with the correct fluid.
Don't drive it until you suck out the extra fluid. A suction gun is the tool you need.