First put in the master code. When you have low life press start and then press A. When you unpause the game you will have full health.
Enable the master code. Pause the game at any time and then press C. Three numbers will appear, the bottom being the level number. Press Left and Right to change the number and unpause the game to start at that level.
Before using any codes you must first input this one. At the Title screen, hold A + B + C + Start on both controllers. If you've done it properly a chime will sound.