Changing your transmission fluid is more involved than just changing your engine oil.
To change transmission fluid you need to remove the transmission pan and gasket by removing all the bolts to the transmission pan.
On some cars this is very hard to do because it may involve removing a transmission mount and then jacking up the transmission or even removing a cross member or something else that is in the way.
Then when you get the transmission pan off you need to remove the transmission filter and replace it as well and then put a new one in and a new transmission pan gasket as well and make sure it seals properly.
Then put everything back together again and add transmission fluid.
If you've never done it before be prepared to devote some time to the job of changing transmission fluid.
Some cars you may need to have the car on a lift to even get at the transmission fluid pan so sometimes it's best to just take the car to a transmission shop and let them do the job.