What type of light? If it's something like a 400w or larger MH or HPS then you need to open the transformer case and rewire at the transformer tap. If you can rewire it there will be labels on the wires that say 240v or 120v on the input side. Just swap them out, neutral is neutral so that is the same there should only be one wire that you need to change. It's either a wire or a tap.