When multiple passes are needed for a clean shave or the skin feels slightly warm after use. push the raised point on the back of the old head forward with your thumb, then press the new head down into the base latches. soak the detached head in soapy water briefly, use a soft brush to clean between the foils, and rinse with clean water. check the mold number on the head holder to confirm it matches those listed. 3 heads per box, enough for three years with one replacement annually.