The transmissions used in that model year (4T65-E and 6T75-E) are sealed units. They are designed to be filled and serviced through a specific dipstick tube which also acts as a filler tube.
To change the transmission fluid, you'll need to remove the pan and filter, which will allow the fluid to drain out. Then, you can refill the transmission through the dipstick tube.
Important Note: Changing the transmission fluid yourself can be complex and requires specific tools and knowledge. If you're not comfortable with it, it's recommended to have a qualified mechanic perform the service.
