Changing transmission fluid can be a very messy process. Mechanics and automotive shops use a transmission flusher, which completely flushes the entire system without removing the transmission pan. Unless you have access to the expensive equipment or are willing to pay a mechanic to perform the service for you, you will have to do it the manual, messy way.
You are going to have to lift the car to get to the transmission. A jack or a couple ramps will work. Put wheel stoppers behind the rear wheels to prevent slippage.
The transmission fluid needs to be at normal operating temperature, so turn on your engine and let it run for a few minutes while in park. Turn it off before you begin your work so the vehicle is cooled to where you can work with it but the fluid is still warm.
Find the transmission pan, which will be sitting just below the transmission fluid dipstick. Some cars require that you remove the engine’s oil pan before you can get to the transmission pan. Other cars have exhaust system parts in front of the pan. Once the transmission pan is exposed, you can drain the fluid.
Some pans have fluid plugs that allow you to simply drain the fluid out of the system. In other cars, you have to remove the entire pan. Once you start to loosen the bolts and remove the pan, the fluid will leak out. Be prepared to catch the leakage.
To avoid an overwhelming mess, only completely remove the bolts on one side and loosen the bolts on the other. Then tip the pan sideways into whatever you are using to collect the old fluid. Once the fluid stops spilling over, take out the other bolts, remove the pan, and tip out the rest of the oil. If possible, measure how much fluid came out so you know how much needs to be replaced.
Remove the old transmission filter and throw it away. Before you put everything back together and add new fluid, you need to clean the pan. Once the pan is clean, install a new filter, and put everything back together.
You can now add more fluid through the dipstick under the hood. If you measured how much fluid you were able to drain out, put a little less than that in and turn your engine on. Once it is hot, check the levels and add more fluid as needed. Be sure to not over- or under-fill the pan.