Any PSU with Active PFC should be autoranging, and thus require no switch. Just get the appropriate European power cord and plug it in. If your PSU doesn't have Active PFC, there should be a small (usually red) switch on the back which toggles between 110/220V or 115/230V. Set it to 220V or 230V when you get to Europe. Make sure it's set correctly, as if you forget to change it when moving to Europe it will very likely destroy the power supply.