Set number of lives:
Load or reset the game, then execute the following BASIC commands before running or restarting the program. Note: X represents the number of desired lives. 

POKE 5363,X
SYS 5000 

Unlimited lives:
Load or reset the game, then execute the following BASIC commands before running or restarting the program. 

POKE 5605,76
POKE 5606,31
POKE 5607,22
SYS 5000