It is recommended generally that fluid is changed every 30,000 miles. You can do it yourself or go to a garage or mechanics. It will likely cost you around $30-150
Its usually done on 30k miles maintenance. $60 for labour for a complete flush and refill of the transmission fluid. Transmission fluid itself cost less than $100. It will vary from car to car.
Generally speaking, transmission fluid doesn't have to be changed. It just has to be kept full. If you have to, it can range from $40-$200 depending on car make.