For standard automatic transmission fluid change, you would drain the fluid from the drain plug and add new fluid. This method will only replace about half the fluid in the system as the remaining fluid is still sitting in other components of the transmission.
An automatic transmission fluid flush would entail either using a machine to extract all the old fluid while replacing with new fluid (this will use about double the amount of fluids as a standard drain and replace). Alternatively without the use of a machine you can drain and replace the fluid, run the transmission, then drain and replace a second time. This method is not quite as effective at removing all the old fluid as a flush machine.
In the case that the transmission fluid is extremely dirty or you are having some other transmission issues it may be a good idea to remove the transmission pan and replace the internal filter (if any), as well as clean the magnets that attract tiny metal particles.