I’m going to second asemechanics advice, drop the pan, replace the filter and refill every 30,000 miles. Do that and you never need to flush or “fluid exchange”. Even though you don’t replace all the ATF this way, you replace enough to keep it fresh.
If you do a “fluid exchange” or flush without first dropping the pan and changing the filter and filling the pan with fresh fluid, then you will NOT replace 100% of the ATF either. You will only dilute it, its about as effective as just a drain and refill. But you only need to do the 100% ATf exchange if you have been severely negligent in servicing the transmission.