Pause the game, and press 'M'.  You should hear a laser gun 
sound. Type 'MOTHERF**KENKIWIB**TARDS' (fill in the stars...) 
Unlimited lives are yours. 
 
Or, on level 2.4, stand at the entrace to the room with the 
giant octopus is located and fire to the right a few times.  
A warp will appear that will take you the beginning of level 3.1.