From title screen, head into the "Continue" menu. There, press the following sequence of buttons - B, Right, Start, Right, Right, Select, Right, Left, Right, Right, Right, Left, Select, Left, Right, Select. A two-digit number will appear in the lower-right corner. Use the A Button to cycle through the numbers, which scrolls from 00 to 1A - these represent the levels from Level 1 (00) to Level 25 (19) in hexadecimal code. The final option, 1A, will simply restart the game as normal, but without background music. To activate whichever option or level you want, once it is represented in the bottom-right, press the B Button and then load a file.
