What is the procedure of resetting the flexible service indicator on a 2006 R350?

Its on the stick at the top of the forum, follow the link and go down the list of documents and select the one dated 3-12-2009.

Here is what it says for the R-class

Reset Service Indicator
1. Insert key into ignition, switch to position “1“.
2. Press buttons or repeatedly until the mileage is displayed within the
Multifunction display.
3. Press cluster illumination button (1) 3 times in short succession. The battery
voltage will be displayed
4. Press the up / buttons until the service menu appears.
5. Use the and buttons to select menu option “Confirm“.
6. Press button until “Complete Service” is selected.
7. Confirm “Complete Service” with button . The display will show “Service
Confirmed”.