or you can try this

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