Philips Norelco HQ9/52 Replacement Heads fit the following shaver models: AT900 series, 8100 series, 8200 series, 9100 series. To replace the heads: 1) Switch off the shaver and disconnect it from the power outlet. 2) Press the release button to open the shaving unit. 3) Turn the lock anticlockwise and remove the retaining frame. 4) Remove the shaving heads and place new ones in the shaving unit. 5) Make sure that the projections of the shaving heads fit exactly into the recesses. 6) Put the retaining frame back into the shaving unit and turn the lock clockwise. 7) Close the shaving unit until you hear a click. Note: If the shaving unit does not close smoothly, check if you have inserted the shaving heads properly and if the retaining frame is locked.