The marking is on the side of the switch and there are only two settings.
The switch can only moves toward one end or the other. So simply move the switch toward the 115 setting.
If you turn it on by mistake with the switch to 230V, it just won't work but the 110V AC will not blow the power supply. That's why all the units are shipped with 230V setting so that people in higher voltage countries will not blow the device.