The simple answer to this is to add more sugar. The yeast eats the sugar and that produces more alcohol. Most brewers will use dry malt extract as their sugar source because it will add more alcohol to the beer, but doesn't add a lot of sweetness to the beer like table sugar will. Keep in mind the yeast can only handle so much alcohol, so be careful on how much DME you add.
As the alcohol level rises in the wort, the fermentation begins to slow down. Adding yeast nutrients to the wort can give the yeast new food allowing for an extended fermentation period. Yeast nutrient also helps to create stronger cell walls, which make yeast less susceptible to alcohol death. Another way to increase the alcohol level in the beer is to add yeast with a higher alcohol tolerance towards the end of fermentation.
Recipe Kit add-on ingredients- 1 lb. DME will add about .5% alcohol
- 2 lb. DME will add about 1% alcohol
- 1 lb. Brown Sugar will add about .9% alcohol
- 1 lb. Maple Syrup will add about .7% alcohol and will add flavor
- 1-2 lb. of honey will add about .7% alcohol and will add flavor