At the title screen enter the code to access the password screen.
The following passwords can be used:

PC-ENGINE (message)
NAMCO (message)
NAMCOT (message)
6502 (message)
6809 (message)
68000 (message)
756-2311 (message)
YAMASHITA (message)
AKIRA (message)
KOMAI (message)
KAZUHIKO (message)
KAWADA (message)
SPEED-UP (4 way split screen) reset after entering the password
S.62.08.22 (Start from level 5) reset after entering the password

Code - Effect

Hold I, II and Select and press Run - Password Screen