For Standard version tank:  

  1. Unscrew and remove the top cap; 
  2. Pull down the airflow control ring (see highlighted part in below picture), the used coil will automatically pop up. Push in a new coil in place;
  3. Screw the top cap back.

For Child-proof version tank:

  1. Press and unscrew to remove the top cap; 
  2. Pull down the airflow control ring (highlighted part in below picture), the used coil will automatically pop up. Push in a new coil in place;
  3. Press and screw the top cap back.

Note: The pressing and twisting operations in each step are performed simultaneously.