Depends on the vehicle. My service spec says transmission fluid and filter change every 75000 miles. They have to drain, drop pan, change filter, install new gasket, replace pan, then add new fluid. To remove trans-axle pan, a frame cross-member has to be dropped. $200 service job (ouch!). If you can get by with only a fluid change, it will be much easier (and cheaper).

The pumping method also removes the old fluid from the torque converter, just dropping the trans pan won't drain the fluid in the torque converter. If the fluid is damaged the pumping method is preferable to removing the transmission to access and drain the fluid in the torque converter.

Go here and scroll down to the step-by-step guide.

As has been stated by Bwire and others, there are two ways to skin the cat, but neither by itself is a panacea, each has their strengths and weaknesses. A transmission flush will actually put a machine in place of the transmission cooler connections that uses the transmission's existing fluid pump to pump the old fluid out and pump fresh fluid back in it's place. Usually they flow two or three full volumes of fluid through the transmission to dilute the contaminated fluid really well. If you really wanted to be thorough you'd replace something closer to 10 volumes, but nobody ever does and that is a heluva lot of oil to chuck. If you really wanted to do a good job, after replacing a couple of volumes you could then switch out the machine and replace it with what is essentially a hydraulic flushing filter cart, filtering the fluid in a closed loop with a really fine filter, but again nobody ever does. But this will not remove metal shavings from the pan, nor will it replace the pickup screen. Dropping the pan will allow you to replace the screen and clean out the shavings, but it won't address the contaminated fluid in the torque converter or in the valve body. Ideally you could do both, but it would be expensive and really wasteful of transmission fluid to boot. so the best compromise is to alternate a transmission flush and dropping the pan at around 60k intervals. Now if the transmission has NOT been maintained, then flushing the transmission might do more harm than good by stirring up the shavings and redepositing them in the delicate hydraulic valve works. Dropping the pan is the best course of action then. One word of warning, most transmission pans do not have drain plugs, so dropping the pan can get pretty messy. If this is the case on your vehicle, you might do well to get one of those hand drill powered centrifugal pumps or the squeeze bulb siphons and drop the suction tube down the dipstick tube and suck as much fluid out as possible before you drop the pan. Otherwise, you'll end up wearing the contents of the pan in your hair (and/or beard). Take my advice, your barber will thank you. =D

I normally pull the bottom of the fill tube from the trans and siphon the fluid there but to each his own.

This week I'll do this to my vehicles and dump the fluid into the oil barrel that feeds fuel to the furnace, mixing with 200 gallons diesel, used motor oil and various vegetable oils

Read this entire thread previously posted about ATF

http://cr4.globalspec.com/thread/37121/Best-Way-to-Change-Transmission-Fluid

And now for the politically incorrect way.

My transmission is high mileage, about 200,000. I noticed some time ago that I had a rear trans seal that leaked. I let it leak for a couple of months, adding oil as necessary. Eventually, I changed the seal and the destroyed rubber trans mount and now I have reasonably fresh oil in a leak free trans AND a new rear mount.

Would poking a hole in the rear seal be thoughtless and irresponsible? Sure! But for the cost of a seal, about $5, a rear mount that probably needed changing anyway, about $10, and a few quarts of oil which I would have needed anyway, I'm guessing 6-7, I got the job done with very little mess. I don't have any shifting problems so I figure the tablespoonful or so of particles spread over the large surface area of the filter hasen't been enough to become an issue. And when it does, it's rebuild time anyway.