Alcohol Content

You can add either Dry Malt Extract or Liquid Malt Extract. But it will change the balance of the beer, so you also may need to add a proportionate amount of hops for balance. Rather than make beers from kits, there are lots of higher alcohol extract or partial extract recipes on the Internet that you can try. Basically the same as a kit, but you purchase the ingredients separately yourself. It's a first step towards eventually making your own all-grain beer.

 

Expanding upon @OldBrewer's comment: You need more fermentable sugars for the yeast to convert to alcohol. As suggested, you should look into extract or partial mash recipes and purchase the ingredients from a local homebrew store (if there is one in your area). This should not only improve the quality of the beer, but it will also give you the ability to find a beer that meets the style and alcohol content you're looking for. You'll learn more along the way as well! Cheers

 

Yeast turn sugar into alcohol. More sugar thus equals more alcohol. Sugar can be derived from malt, table sugar, corn sugar, fruit juice, etc...

As warned above, you are changing the beer by doing this, maybe for the better but likely for the worse. You are also likely to have better luck with malt extract than sugar if you go this route. Also as mentioned, you will likely need to add more bitterness (hops) to counteract the additional residual sweetness.

 

It's basically how beers get 'big'. Permit a dramatic oversimplification.

A pale ale has less alcohol than and IPA. It uses less malt and hops than an IPA.
An IPA has more alcohol than a pale ale. It uses more malt to provide more sugars to ferment into alcohol. Therefore, it also uses more hops to maintain the balance and preventing the beer from getting too sweet from the increased malt bill.

That takes things down to the most very basic aspects. It's not just as simple as 2x malt = 2x alcohol = 2x sweetness and therefor 2x hops need to be used (although Dark Horse Double Crooked Tree is a good example of this being done with some success where they literally just doubled their Crooked Tree single IPA recipe).

Bottom line is if you want more alcohol, use more malt (or other form of sugar). But you'll almost certainly have to adjust your hops, and therefor will have a different (even if slightly) beer.

 

If your brewing kits, check the abv. If all grain what the earlier posts said.

 

Keep in mind when adding extra ingredients in order to make a 'bigger' beer, the entire process becomes more complicated with a need to be certain that you properly aerate your wort and choose a yeast that will work okay with the expected higher alcohol. Since it sounds like you are a newbie at this hobby, I suggest that you don't go overboard with this 'adding sugar' process until you've gotten your basic brewing procedures established so that you can minimize any mistakes.

 

As @GormBrewhouse mentioned (or at least implied), there are plenty of kits designed to make high ABV beers. You don't have to buy a low ABV beer kit and goose it with extra sugars.

 

You can always boil down your recipe to a smaller proportion. I.e. Boil your wort for 30 min before beggining the hop schedule and from that point do it normally. You'll end up with a more concentrated wort, darker color, and some more carmelization. But usually this method will be about the same taste because the excess sugar will be fermented out, and your body a bit thinner. I've done this before many times, mostly to experiment, and with excellent results.

 

I'm with the guys who are advocating for buying kits that make bigger beers to start. Make sure you pitch yeast appropriately!