How do you change the transmission oil in corolla S 2003?

There should be a drain plug on the pan. if not, you have to drop the pan. you should change the filter at the time of a fluid change anyway which is just inside pan & only 1 bolt generally to attach. you may have to support tail shaft & move cross member slightly. You will also need a trans pan gasket & torque in sequence to specs. When done just refill when running to proper level on stick.