How do you replace 120 volt pool light with a 12 volt pool light?

Answer for countries in Europe and other world areas running a 50 Hz power supply service. This depends on the the light or globe, if you can replace just the globe its very simple. change over the globe and buy yourself a mains(230v or 115v) to 12v isolation transformer, and attach that to the mains cable and run a cable from the 12v to the globe and away you go. The description of how to change the mains to 12v is the same if you have to replace the whole light. Then you just have to replace the globe or the whole light assembly.