Well, with due respect, that's a confusing part. when and how to treat with the transmission fluid. whether its need to flush out or just changed. But let me try.....
The best thing is to check the car's manual (if have one). Experts say it is better to change between 60k - 100k mileage or to check the oil condition manually. I am attaching the image from the internet that will hopefully make sense to understand that when to change gear oil.
For auto transmission, to check gear oil condition, take fluid dipstick when the car is warmed up.
Hope it helps.
Regards,