If you want to do a true fluid change this is what I did.
Buy your filters and Mag Hytec pan from Geno's and buy 5 gallons of AMZ/OIL all purpose ATF.
Now remove the pan and filters, be sure the O ring comes out. Loosten the valve body bolts, this will get more fluid to drain. I let mine drain overnight.
Next day, install the filters and the O ring. Tighten the valve body bolts. Install the new pan. Measure how much oil was removed. Now add that amount back into the transmission, now you are back to FULL.
At the front drivers side near the radiator are two steel lines with quick connect fittings. The top one is the feed line, disconnect it. Use a 5/16" x 4-5' fuel line hose and slip it into the steel hose connection, make sure the hose fits snug or tape it to seal. Have at least a 10 quart container to drain the ATF into. Have someone start the engine, be sure to keep the hose from coming out (don't ask me why) run until the hose starts to sputter. Measure the amount discharged. Add the same amount back. Repeat the same process until the fluid color changes, you will notice. You should know what the total fluid capacity is so you will know when you are close. When the color changes I drain about an extra quart. Re fill the same amount drained. Re connect the line. Start the engine and do a quick level check. Take your truck for a spin to fully warm the trans up come back home and park in a level spot and adjust the level if needed.
While you are at it drain the power steering pump, the two hoses are at the bottom of the radiator. They have spring steel clamps, slide them back on the hoses and remove and let drain. Measure and re fill the same amount. It also takes ATF, I used AMZ/OIL.
This method works for the 2010-2012 trucks.