Caution: Drained coolant cannot be reused. Coolant is poisonous. It must not be poured out anywhere, especially not into household waste.
Drain
Set the heating switch in the car interior to the maximum heating position.
Place the car on jack stands.
Remove the lower engine compartment cover.
Open the expansion tank cap.
Caution: If the engine is hot, cover the cap with a thick cloth before opening it to protect it from steam emissions. The cap may only be removed when the coolant temperature is below 90°C.
4-cylinder engine
Place a clean container under the water pump.
Loosen and move back clamp -1-, remove hoses and completely drain coolant.
5-cylinder engine
Place a container under the engine.
Lower the car, remove the heating hose from the rear of the engine block.
Loosen the hose at the bottom of the cooling system line
Filling
Put on the hoses and tighten the clamps.
Install the lower engine compartment protection.
Lower the car.
Open the heater control completely.
5-cylinder engine: Open the air bleeder plug and wait until coolant comes out.
Fill the coolant to the mark on the expansion tank. Close the tank.
Start the engine to turn on the fan.
Check the coolant level, if necessary, top up slightly above the mark. When the engine is warm, the level should be slightly above the Max mark, when cold - between the Min and Max marks.
(A link to the original source is available on the website: AudiManual)