Do I just change the transmission fluid or do a transmission flush

Hello, I'm Danny.

First thing is to check the condition and smell.Make sure the fluid is not brownish/black in color and doesn't smell burnt.The best way to service is remove pan to change filter and fluid.When places offer a flush they just tap into the transmission cooler lines and just exchange the fluid and leave the old filter in place.Here is a tutorial showing what is involved:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

I've attached a picture below showing your trucks transmission requirements and how to steps.Hope this helps and thanks for using 2CarPros

Hello again.

Yes, just drain fluid from drain plug and remove pan when done draining. Replace filter with new one. Reinstall pan and then fill with new fluid. Hope this helps and thanks again for using 2CarPros.

Danny-