Purchase a new strap suitable for your make, size and model of helmet; not all straps fit all helmets.
Locate the screw or button attaching the chinstrap to the helmet -- it will be at one side of the helmet.
Unscrew or unbutton the old chin strap and discard.
Button on screw the new strap where the old strap was buttoned or screwed. Tighten the screw, if applicable.