Sugar contains about 45 points of gravity per lb. One pound of sugar in a gallon of water will raise the gravity by 45 points. In a 5 gallon batch, it would raise the gravity by 9 points (45 / 5 = 9).
A drop of about 7 1/2 points of gravity is equal to approx. 1% ABV.
You want to raise your ABV by 1%, so you need to raise the gravity by 7 1/2 points. Multiply batch size (# of gallons) by 7 1/2. Divide by 45 and you'll get how many lbs of sugar to add.
It would be better to add the sugar prior to fermentation. Doing it post fermentation, you don't want to stir the cider and oxygenate it. I would dissolve the sugar in a small amount of water. Heating the water will make it easier. Then, add the sugar water to the cider.