For a kit - to make it weaker add more water.
But, If you dilute it with water, it'll be thinner in body, and will be less bitter and have less hop flavour (unless you add more). There are ways of adding body as well - such as maltodextrin.
To make it stronger add more fermentables - preferably dry malt rather than sugar.
If you add more malt, then you'll have the same hoppiness/bitterness, but a more malty flavour, so the balance won't be quite the same. But not necessarily worse!
If you add sugar, it'll be stronger. But my opinion is it won't taste as good as adding malt. But then I don't like strong alcohol, low body beers - just my preference :)