IMO, the best way to change out the fluid is to pull the rubber return line that comes from the cooler and pump all the old fluid out that way. You get a much better fluid replacement that way, instead of just dropping the pan and replacing what little fluid is in the pan. To pump the fluid out....pull the line, put it into a big bucket (so it won't come out when the fluid starts pumping), start the vehicle and shift into drive, when the fluid stops pumping, shut off engine and add some new fluid to the tranny, repeat previous procedure until the fluid that is getting pumped out is clean fluid. Once clean fluid is being pumped out, drop the pan, clean the pan, change the filter, put a new pan gasket on (unless it is a rubber pan gasket that is still in good shape, then you can reuse it), reassemble everything (don't forget to hook the line back up that you pulled to drain/pump the fluid out), and refill the tranny. Warm up tranny and recheck fluid level.
I have used the above procedure for a tranny that was not shifting correctly and it fixed the problem. I believe my problem was a gummy valve body and it eventually got flushed out and started operating correctly, once I got some new fluid in there.