There should be a Torx head bolt on the sump pan under the autobox to drain the fluid. And then another Torx (or sometimes an Allen) head on the side of the box. Different types of automatic transmissions were fitted so the filling point can be on the left or the front- or rear-right of the box. It's worth measuring how much fluid you get out. Put the drain plug bolt back in and fill with automatic transmission fluid to the top. Start the engine and, with your foot on the brake, move through all the gears before putting it in 'park' again. As the gearbox oil heats up, you should get a small amount of fluid leaking - if that's the case, the fluid is at the right level - if not, top it up. There's a huge debate about which kind of fluid you should use in these units - the best advice we can give is make sure it meets the spec of your vehicle and buy the best you can afford. If in doubt, contact your local BMW dealer to check fluid requirements. It's probably worth changing the filter while you're doing this, and you may need a replacement washer for the drainplug. For this job, the car ideally needs to be on a ramp or axle stands - so please make sure this is done correctly and the vehicle is secure before going underneath it. If the light isn't very good, a head torch will improve things and leave you with both your hands free to work on the car.