You can drain it a few ways. If you are only looking to change about half the fluid, you just drop the pan. I'd recommend taking out all but one of the bolts and easing down a corner on the far side to keep the fluid from flying all over the place. If you want to drain the entire system, then unhook the pressure side hose that runs to the trans cooler and then turn on the car. It will spit out as much as it can and then you have to drop the pan.
Fill it up through the dip stick.
A pan drop should take about 3.5 quarts of fluid, a full empty will take about 7.