Where can the input voltage selection be changed from 110 volts to 220 volts on a computer system?

The voltage can be changed for a computer in a few ways. The advanced way is through the BiOS. However this is not recommended for an everyday user! You can find a few programs online that will allow you to safely change the voltage and some that is still for advanced users but easier to do.

WARNING! Changing the voltage can permanently damage your PC. Most likely cause overheating. You should never change the voltage on a PC unless the goal is to overclock! Overclocking will void computer manufacture's warranties!