Start at 1875 (Part 3):
Hold A + B at the title screen to display the password menu. Enter FLUXCAPACITORISTHEPOWER as a password and press Select, Start.

Hint: Access rooms:
Occasionally, rooms will appear that seem impossible to reach. If they are too high for your character, try going back in time and planting an acorn under the desired location. After returning to that spot in the future, a large, healthy tree suitable for climbing should be available.