Labo VinCorrecting the sugar
All you have to do is adding sugar, first dissolved in a little juice (otherwise, it will form a layer at the bottom of the carboy). As sugar, sucrose (beet, cane sugar) is ideal. It is cheap, has a neutral taste; it is readily assimilated by yeasts (which has first to hydrolyse it in a mixture of glucose and fructose, but they excrete the suitable tool: invertase).
Glucose (grape sugar) is suitable, but more expensive. Honey can also be used, but it will give the wine its own taste. 1.4 g honey equals 1 g sucrose.
When a large amount sugar has to be added, it must be added in several times: by 2 then 1% potential alcohol from 12 % vol.