Easy, add more sugar. Most brewers prefer a dry malt extract (DME) for their sugar needs. Table sugar is not a good choice, it will give your beer off flavors. Every pound of DME used will raise the alcohol by a half percent. Be careful not to add too much. Too much sugar will shut the yeast down from working. Maple syrup, brown sugar and honey are all good choices for sugar sources. They will add alcohol and flavor to your beer.