When the blades feel sluggish during operation or stubble remains when wiped with tissue, replacement is due. push the old head toward the foil direction to release from the clip, then align the new head and press into the slot. use a cotton swab with alcohol to wipe the foil edges, rinse with water, let dry, and reattach. check the model number on the head holder or razor base to confirm it appears in the list. 3 heads per box, enough for three years with replacement every 12 months.