Assuming you are talking about a motorcycle helmet, the best way to clean the visor is to first remove it from the helmet. Once it is off, you can use a mild soap and a soft cloth to wipe down the outside of the visor. Be sure to rinse it well so that no soap residue is left behind. To clean the inside of the visor, you can use a mild cleaning solution and a soft cloth or sponge. Be sure to rinse the visor thoroughly after cleaning it.