Extract batches tend to finish with a relatively high FG compared to similar all-grain beers (higher FG means a sweeter tasting product). This seems to be a major complaint of extract brewers. Adding some simple sugar will both increase alcohol and lower the FG. Adding 1 lb of table sugar to 5 gallons will increase abv by ~1.3% and lower the FG by about a point.
Be careful using too much sugar. It not only dries out the beer, but lowers the body, and thins out the beer. I would not exceed 10% of the fermentables as sugar in most beers (Belgians are an exception). You can use most any sugar, white, brown, Turbinado, honey, corn sugar, etc. Both corn sugar and honey have less fermentable sugars than plain table sugar. Do not use powdered or confectioners sugar, as it contains cornstarch. Cornstarch will not ruin the beer, but will probably make it cloudy.
If concerned about the loss of body, use roughly 2/3rds extract and 1/3rd sugar, and that should leave the FG and body similar to the original product.