At the title screen press C-Up four times, C-Right six times, C-Down, C-Down, C-Left, C-Left, a scream will confirm the correct entry of the code and enter the options screen to access more difficulty settings.
On the Title Screen press Up, L, Up, L, Down, L, Down, L, Left, R, Right, R, Left, R, Right, R, B, A. Then hold Z and press Start.