This is a writeup on a great method by a member here.
http://www.truckmodcentral.com/forums/f63/how-flush-your-transmission-gmt800-gm-truck-4699/
This method allows you to change all the fluid, Not just what`s in the pan. It`s very easy to do.
The more difficult task is removing the shift linkage in order to drop the transmission pan. The bolts are torx and are tough to remove because you can`t see them. You have to find them by hand and if you live in the rust belt it will be more difficult to loosen them. I couldn`t get them out so i just bent the bracket down and was able to get the pan down. I didn`t bother bending the bracket back because it didn`t affect the shifting at all.
And when you go to buy a new filter you`ll find out that there are two different pans for our trucks, You either have a deep pan or shallow easily to tell apart by getting under there and looking. The shallow pan is flat across the whole bottom where the deep one drops down about half way across.
I also reused the oem pan gasket with zero problems.
And welcome to the forum.