With the game off, locate your MPU in the middle left hand quadrant of the backbox. On the MPU you will see a similar chip (ROM) on your MPU hopefully marked L-1. Now some people can do this with the board still in the game, others have to take the board out to the work bench. This is for you to decide based on your skill level.
You can purchase an IC puller from Radio Shack or use a very small jewelers screwdriver to gently and SLOWLY pry up the ROM from the MPU. You do not want to bend the legs of the IC or damage the socket or gouge the circuit board. Take note of the orientation of the little notch on the IC that you are removing. You will want to install your new L-4 ROM in that same orientation.
Installing the new ROM involves making sure that ALL of the legs line up with each spot in the socket. You may have to GENTLY squeeze the legs towards the middle of the IC to get a perfect fit. Go slowly.
Once you have verified that you have the legs lined up, you can apply gently downward pressure to seat the chip in the socket. Again go slowly, you don't want to bend the legs or break them off. Once the chip is fully seated (reinstall the MPU if you took it out of the game) you can turn the game on and enjoy your upgrade!