In most of these GM 4 speeds, you do indeed have to remove the pan. You loosen a few screws on one side of the pan, and begin to remove them slowly to begin the drain on a small section. It is best to remove them on the side of the most gravity if you have a large drain pan. You can begin to remove them around the pan to allow you to drop it even more in the draining side. If you do it right, you won't splash any atf. Clean the pan and remove steel shavings and any sludge material. The magnet should be cleaned and placed back where to found it, which is generally marked in the OEM pans. Filter and O ring should be replaced. You can gently tap the old O ring inward with a flat screwdriver and mallet. But don't damage the transmission. When the O ring is bent inward a bit, it is easily removed with pliers.
Many of the pan gaskets with metal rings in the screw holes are allegedly reusable, but I like to replace them anyway. Get an OEM style gasket. Don't use the cheap rubber things (and never use cork!) or you will have to overtorque to keep it from leaking and might damage the pan around the screw holes. You can find a few inexpensive OEM style filter and gasket kits on Amazon for around $25, which is the same price that auto parts stores will charge for one with an inferior rubber gasket. The gasket needs no adhesives, and I believe that the pan is torqued to about 8-9 ft lbs when wet.
I generally replace the filter and refill in my GM cars with these transmissions every 40 to 60k.
It is very important not to overfill. In these 4 speeds I add 3.5 or 4 quarts before starting the engine. Let it warm to operating temp for 5 minutes and add more fluid with the check plug removed. You should cycle throgh gears a few times in the process. If ATF begins to drip out of the check plug, it has been ovefilled. Let the dripping cease before plugging it back up. ATF should only be filled on a level surface.