One good method:
1) Drain trans fluid
2) Remove trans cover
3) Remove old filter, and a new one
4) refil with fluid
Another that get more fresh fluid in the trans
1) Disconect inlet and outlet hardlines
2) Slip rubber hoses over the ends of the hardlines
3) Place the return line in a gallon of new trans fluid
4) Place the opposite feed line into an empty container
5) Start engine, old fluid will be captured in your empty container, new fluid takes its place. Be sure you have enough new fluid and enough empty capture containers.
6) Allow engine to run untill fluid coming out of the feed hose is fresh fluid.
7) Reconnect rad/trans connections
8) Cycle the trans gear selector with the engine running, leave running.
9) Check fluid level, top off as need.
peace
Hog