If it's an analog speedometer, first check the condition of the speedometer cable. It may need to be replaced.
For the FSS Reset here it is (i copy pasted this from rudeney from another thread) With the engine stopped, turn the ignition switch to position 2 and within 5 seconds, press the odometer reset button twice. Turn the ignition switch to position 1, then press and hold the odometer reset button while turning the ignition switch back to position 2. Continue to hold the reset button until the buzzer sounds and/or the new service interval is displayed on the odometer.