How do you reset service indicator message for 2000 Mercedes Bens E240 after b is complete?

Most likely the same as the 2000 E430 which is as follows:

turn key to position 1

press up or down arrow til service indicator appears

press and hold (R) button for approx 2 seconds

display will show "do you want to reset service interval?"

press (R) button again to reset

Hope that works for you.