Changing Transmission fluid in a Jeep Liberty?

[I love carbs yall]

Is Transmission fluid something that should be CHANGED OUT or can it be added to? Like buying a funnel and pouring a quart of new fluid in?

I want to make sure my fluids are good, but I need to get some transmission fluid, but I don't want to mess anything up.

I have a 2004 Jeep liberty, and if it needs to be changed, is it hard to do? And if i take it to a shop, how much do you think it will cost just for them to do this?

I live in Pensacola, FL.