I believe this is the method i've used before to reset it, real easy.
Service light reset procedures :: Gendan Automotive Products
Audi A4 & A6 (All years)- Turn off the ignition
- Press and hold the button on the left side of the display while turning the ignition on
- Service OIL will be displayed
- Pull out the button on the right side of the display until the service prompt is cleared
- The display should now read ---
If the above procedure does not work, outlined below is an alternative procedure: - With the ignition off, push the right button
- Turn the ignition on and then pull the left button
- This should clear the warning message