It depends on what size batch you are doing. I recently helped a friend bump up the ABV of a Mr. Beer kit he got for Christmas. For a two gallon batch, we added 1 pound of light DME.
I suggest you use a recipe calculator (http://beercalculus.hopville.com/recipe) and enter your extract kit. Then determine how much malt extract to add to get your desired beer. For an IPA, you will probably also want to add some hops to balance the additional malt. Finally, consider your yeast. If you bump the ABV too high, it could be too much for the yeast provided in the kit. Most likely not, but it wouldn't hurt to have an additional packet of yeast on hand. The Mr. Beer kit I helped my friend w/ did not have yeast info so we just pitch some US-05 in addition to the yeast in the kit.