The fluid in an automatic transmission not only lubricates the transmission but is also used to transmit power from the engine to the driveline. It's important to follow the manufacturer's fluid recommendations and maintenance schedule to keep your transmission operating smoothly.
There are three different methods for changing the transmission fluid, two of which you can do at home. Before you start make sure that you have enough of the proper type of transmission fluid on hand, the proper filter (if you plan on replacing it), and a transmission pan gasket (if you plan on dropping the pan).
The traditional method used to change the transmission isn't actually a flush, but a simple drain and refill that will only replace about 50% of the fluid. A few vehicles provide a drain plug that makes this process easy while others require you to remove the pan to drain the fluid. Once the fluid has been drained the filter can be replaced. Some cars have an externally mounted transmission filter, however usually the filter is mounted inside the transmission and is only accessible once the pan has been dropped. After the filter has been replaced you can reinstall the transmission pan with the new gasket. Slowly fill the transmission fluid, usually though the dipstick tube, being careful not to overfill. When you have close to the proper amount of fluid in the transmission start the car and let it get up to operating temperature while checking the fluid level. Check for leaks and then take the car for a spin to verify that everything is working properly.