I'm not sure which locomotive you have, but if it is the scale Berkshire with Legacy (Lionel #6-11203 or 6-11146 for example), the switch is located underneath the locomotive just above the right side trailing truck wheels.
Below is the procedure for a factory reset if you are using TMCC. The reset procedure using Legacy is different, mainly because in Legacy you can use the memory module supplied with the locomotive to set the various functions.
1. Ensure the TMCC base is connected to the track and all power is off.
2. Slide the "Run/Program" switch on the engine to "Program". You may have to turn the engine upside down and push the front truck to one side to see the switch.
3. Place the engine and tender on the track and turn on track power
4. Using the remote, press "ENG", and then a number (0-99)
5. Press "set". If successful, you will hear the horn sound and/or see the headlight flash.
6. To set the engine sounds and feature type, press Aux1 and then 74.
7. Turn power off, slide the "Run/Program" switch back to "Run"
8. Restore power and address the locomotive - it will now operate normally.
Happy New Year!
-John