I'm no expert but a bit more sugar / maltose / spray malt / liquid malt extract will increase the ABV, but don't go mad or you'll end up with something undrinkable. The kits will have been produced for a balanced flavour for the hops / barley / flavourings they are based on, so you'll upset that by just lumping in more sugar.
Fermentation is the yeast converting sugar to alcohol, but brewers yeast can only withstand around 5% ABV until yeast cells are killed off thus preventing further fermentation, so that's your logical limit.
HTH, happy brewing. We just put 36 pints of bitter to ferment tonight.