You can replace the fluid in your transmission yourself, You just will not be able to replace all the fluid. You will only be able to repace about half the fluid so you will have half new fluid and half old fluid. This is a standard transmission service and any repair shop would do the same thing.
First you need to drain the fluid.
Next, if you would like too, I would recommend removing the bolts holding the transmission pan on and replacing the filter with a new one. The gasket is reuseable so just wipe it off.
Then just bolt the pan up with the gasket, Then install the plug
Next, You will need some sort of handheld fluid pump to pump the new fluid in the transmission.
Warm the vehicle up to normal temperature and with the vehicle running check the fluid level.
I would start out by pumping 4 qts in transmission and then running and checking. It should take around 5 qts.
Hope this helps, If you need anything else, Please let me know.