What kit are you trying to tweak?
Dissolve sugar in a small amount or warm water to make a simple syrup.
Amount depends on how much you want to raise the alcohol level.
The yeast you use for fermentation is rated as to how high an alcohol level it can tolerate. Make sure the total ABV does not surpass this level.
Make sure that total volume of must, with water and added sugar, does not surpass kit's recommend volume. In other words, if it is a 23 liter kit, try to keep the total volume at 23 liters. Otherwise, you will be diluting your wine.
1.5 oz. of table sugar per US gallon of projected liquid raises the ºBrix by 1°. Use the triple scales on your hydrometer to determine how much additional Brix you need to reach desired ABV.
Remember that raising alcohol can (but not always) cause wine to taste like jet fuel for several months after fermentation, so be prepared to age such wine a little longer. This doesn't seem to be a problem for mist kits. It is best to tell us what kit and to ask specifically of those who have tweaked your same kit.