Go to the main menu, hold R and use the following combinations:
2X Speed Boxing: Down, Left, Down, Up, Right, L
Default boxers, arenas & Rocky statue: Right, Right, Right, Left, Right, L
Default Boxers & Arenas: Right, Down, Up, Left, Up, L
2X Punch Damage: Right, Down, Left, Up, Left, L
Max Stats for Movie Mode: Right, Down, Down, Up, Left, L
Max Stats for Tourney & Exhibition Modes: Left, Up, Up, Down, Right, L