The “Flush” machines connect to the cooling lines and “trade” fluid…Nothing gets “flushed” other than your wallet. They don’t necessarily change the fluid trapped in the torque converter either, and they ignore the filter…
So drop the pan, change the filter, and refill the transmission, changing as much fluid as you can…I have started engines with the transmission pan off and let the tranny pump itself out somewhat, 30 seconds or less, no more time than that, others will claim this can damage the transmission, but I think 30 seconds at idle speed is safe enough…
This can be a very messy job, so have a BIG drain pan…