Different manufacturers of helmets make their visor or shield releases in different ways, but the basic idea is the same for all. Each visor clips to the helmet with a release button or tab near the ear area of the helmet, and once found, this makes changing out your visors simple. Sometimes the release mechanism is concealed, so check out the owner's manual for your helmet for specific instructions on locating it.
Rest your helmet on a soft surface so it will not be scratched or damaged.
Open the visor as far as it will go, sliding it up on to the top of the helmet.
Locate the release mechanism. This appears like a thumb tab or button.
Press the release tab or button, one side at a time. The visor will feel loose when this is accomplished.
Snap the visor off by pulling it straight off the helmet.
Replace with the new visor by sliding it onto the helmet in the up position.
Push the visor into the release mechanism until you hear it click in securely.
Replace your visor if it become scratched and hard to see through, or change out for tinted plastic if riding when it's very bright.
If you are unsure about your helmet, take it to bike shop for assistance.