On a manual, it's as simple as draining it and refilling it (you need a squeeze bottle or pump to refill it).

On an automatic, you can only drain about half of the fluid, the rest is in the torque converter and can only be drained by detaching the lower trans cooler line and running the engine. Refilling is the same way, it must be done while the engine is running and you instead pump ATF into the upper trans cooler line. Ideally, you drain and fill simultaneously so the TC never runs dry and stop when the output clears up.
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1980 244 diesel M46 168k, my 4th 240