The setting has to be changed through the center console's charge screen. The kicker is that, to help protect against faulty wiring at various locations, this setting defaults to 8 amps each time you start the car. So each night, you'll need to go press the 12 amps button in that screen. Maybe they'll change that one day, but for now it's the norm on the 2013's
Alternatively, you could buy a 240V charger that will charge the Volt in about 4 hours. They go for $400 and up.