If you leave it as it is you will have a beer of just under 4% (similar to a Wherry kit).
If you want to restore the potential ABV of just under 5% you need more sugars. Unless you have any DME or LME in your store, I suggest a 1lb jar of Holland and Barrett LME and 250g Golden Syrup asap. But don't dump in lots of sugar.
Next your beer will be less bitter than it would have been due to the dilution. There is not much you can do about that, unless you want to fiddle about boiling up hops assuming you have any.
But whatever you do, wait until the fermentation has finished before you add your hops, irrespective of what the instructions suggest.