Only thing you could do is add some form of sugar or some extra extract to it. Since I'm sure you're done with the boil by now, the only way to do this would be to boil the sugar or extract in more water to sanitize and then blend into the wort. I wouldn't bother though. The difference is only one of .656% ABV. With the target gravity you must have been shooting for a Scottish 60/- but this lower gravity is too low for that. However, you will still have a good, drinkable first beer. Better to take what you have here than risk really bombing your first batch. You can work on efficiency later.
I'm wonder, with a kit, if you missed a step or started with too much water, or didn't boil long enough. There are a few other things that might have happened but it comes down to having more water that your sugars are dissolved into than you were supposed to have. For example, if you now have six gallons of wort, boiling down another gallon (to 5 gallons), you would have hit your gravity. This comes through either boiling longer or boiling harder.