What are you trying to do? You can change the voltage of a single circuit by the methods mentioned above. If you are trying to use a LED or other low powered device even an in line resistor will work. On the other hand, and I cannot see why you would want to do this unless you are restoring an old car, the entire system can be converted starting with the battery, the charging system, all of the electrical components including the light bulbs, the switches should be alright. Of course the starter would have to be changed as well, but it could be done, good luck finding a good 6 volt generator however, they are very hard to find. It pretty much means that you have to match the demand for 6 volts to the system you use, if low draw, it can use a simpler and less expensive system, if it has medium to high power draw then a more heavy duty system will be necessary, so it is hard to answer this without knowing just how much 6 volt power you will need in amps.
