There are several ways to boost your abv, adding Malt extracts is one way, adding other sugars (corn sugar, white sugar, maple syrup, etc.) is another. As far as how much, I'd recommend buying beersmith or using a free online recipe builder and that will tell you how much. A rough estimate for DME is about 8 to 9 tenths of a percent per pound of light DME. Though I believe that changes as you get a higher ABV.