To access service mode, hold Job Cancel + Select during powerup until all LEDs are lit, then press the right side of Menu, then Select. If all goes well you should see a message that says SERVICE MODE on the display briefly, then after the printer finishes its startup, you will have a new menu entitled SERVICE MENU. Serial Number will be an option; it is set with the Value and Select keys. Press GO to exit Service Mode once the changes are made.
Hope this helps.