Decanting doesn´t affect the alcohol content of a wine. Decanting simply aerates a wine (or is used as a method to separate wine from its sediment). Aerating a wine might change how a wine´s alcohol is perceived, though. For example, you might find that with some air, a wine seems smoother and the alcohol more integrated; or if it´s exposed to too much air, the fruit flavors might fade and the alcohol stick out a bit more.