Follow this guide to step 18, and you should be able to figure it out from there. To help future fixers; you could create your own guide here on iFixit. Just copy all of the guide linked above and then add the last couple of steps yourself

Hi! For a start, it's probably not voltage you need more of, but current or capacity. If you need more power while the vehicle is running, you will need to retrofit an alternator that can deliver more amps than the one you have currently installed. If you need more power when the vehicle is off, you could either replace your battery with one that has a higher amp-hour rating, or retrofit a second battery wired in parallel. This would double your capacity, letting you run your stereo for twice as long while the engine is off.