Most cars do not have a drain plug on the transmission pan. You would need to remove the transmission pan to drain the fluid. Place a nice size pan under the car and loosen all the pan bolts then lower one side of the pan by turning the bolts out giving time for the fluid to run out.
Before replacing the pan be sure to clean all gasket surfaces and inspect the pan for divets around the bolt holes and remove as needed. when replacing the pan do not over tighten the bolts.