Add more fermentable sugars. One lb of sugar will increase a gallon of liquid by 40 points (a gravity reading of 1.040 if the liquid was water). If you have about 1.25 lbs of fermentable sugar in each gallon you will have a potential level of alcohol of about 7% ABV (alcohol by volume). That amount of sugar will give you a reading of about 1.050 so what you need to do is
Make up the must (the juice or liquid before you pitch (or add) the yeast and measure the gravity. It should be about 1.036 - 1.040 If it is you need to dissolve about 1.5 lbs of sugar in the 6 gallons to bring the gravity up to about 1.050. An alternative would be to add concentrated apple juice (without additional water). You may have to do some arithmetic to determine how much sugar this will add. Adding table sugar will increase the alcohol but not really change the flavor of the cider very much... adding concentrate to the must will intensify the flavor...
A third option is to simply reduce the amount of water you will add to make the kit. In other words, your kit makes 6 gallons at 4.75 % ABV. If you reduced the volume to 4 gallons this would make a cider with a starting gravity of NOT 1.036 but closer to 1.054. That starting gravity will make a cider of about 7% ABV. But all that said, if you have not made cider before (and cider does not involve brewing - you do not heat anything)... then you may want to stick with the kit's recipe as they will have determined the flavor to alcohol balance - That is something you will be changing but if this is your first time fermenting you may be unaware of how yeast and sugar react and how different kinds if alcohols are produced and how aging affects the processes.. wine and cider making is , after all, all about balancing flavor and alcohol and acidic kick...