How do you change automatic gearbox oil filter Audi a6?

there is no dipstick. The level is checked from underneath. There's a drain plug on the bottom surface of the oil pan, and a larger fill plug next to it with a short vertical tube

inside. The tube is the "level checker". You basically have to put in fluid while the car is on a lift or over a pit and perfectly level and keep adding fluid until it runs out the hole. The engine is run (with the car on a hoist) until the transmission temperature is in a very small and specific range, then fluid is pumped into the fill hole (much like filling a diff) until it starts running out. The tube is usually surrounded by a bunch of relays and other crap, making it very difficult to get a straight hose in, so a short tip with an almost 180 degree bend works best. So my advice, get it done by a pro, if you aren't one!