What component increases the alcohol content in beer? I would like to know this for future brewing.
In a word, sugar. In presence of yeast. The yeast acts as a catalyst to a chemical reaction where sugar plus water converts to alcohol and carbon dioxide. More sugar, with yeast in the background, gives more alcohol (and more CO2).
Many people will simply add a pound of plain white sugar to any recipe to add more alcohol. Cheap and effective. You can add any sugar source that you like, including honey, brown sugar, fruit juice, agave.... probably hundreds of sugar sources, and they all work, except for a few oddballs like lactose which is not fermentable.
For recipe ingredients, in an all grain, more grains gives you higher alcohol content. More grains -> more sugars -> more alcohol. Mash conversion is also a factor in all grain. If your mash is subpar, you'll get less of fermentable sugars and more of non-fermentables or even fail to extract the contents of the grains at all. Some key factors could be pH of the water, temperature control, or insufficient enzymes (not enough barley per other grain types).
For extract brewing, more extract = more alcohol.
And the yeast matters some as well. Some yeasts will leave more unfermented sugars than others. This is called attenuation. Yeast that attenuates more will result in higher alcohol content as well as a "dry" (less sweet) beer.
All things being equal though, adding more fermentables (grains, extract, sugar) is the most effective and easiest way to boost alcohol content.
Except for lactose, it is sugars that are converted into alcohol and CO2 by YEAST.
Now your question asks "what CHANGES THE ALCOHOL content of beer" could also be interpreted as, "what happened to my alcohol in my beer?" That would be ACETOBACTER converting your alcohol into acetic acid (vinegar). This happens from gnats and improper sanitation.
dmtailor is correct, a footnote:
Alcohol content is the result of fermentable sugars. Unfermentable (or nearly unfermentable) sugars such as lactose, dextrins and caramelized sugars just hang around and add sweetness, body and (in the case of caramel) color to the beer but don't add alcohol in significant amounts.
Also note that large amounts of refined sugars (cane sugar, corn sugar, etc.) can change the way your yeast ferments alcohol and may cause it to loose its ability to ferment malt sugars (maltose, maltotriose) and cause stuck fermentations. Yeast is best suited to ferment malt sugars and while it will happily handle small amounts of refined sugars, large quantities of refined sugar can have adverse effects.