You have a few misunderstandings here.
1. That is not a 5W laser it is a 5mW there is a big difference.
2. You misunderstanding the relationship between voltage and amperage, the batteries go through the driver which regulates amperage, and it's amperage the controls the output of the laser. So increasing the amperage will increase the output (until you blow it up) voltage will just blow it up.
If you are just trying to increase battery life then assuming it takes 2xAAA tape on 2 C or D batteries to get the same voltage so you won't blow it up but a lot more battery capacity.
If you want to try adjusting the amperage look up POT modding green lasers. Just keep in mind more likely than not you will just blow it up. But you might get some more power. More voltage will just blow it up regardless.
And regardless of what you decide to do please buy some laser safety goggles so you don't blow up your laser and your eyeballs.