The click pen from the Classic Collection is refilled by pressing the small button located on the side above the clip using a pointed object (like a paper clip) and removing the button on top of the pen. Then, simply insert the refill into the barrel of the pen and close the pen, placing the button back in its slot.
The click pen from the Light Metal Collection is refilled by rotating the point of the pen and removing the tip. Then, insert the refill into the barrel of the pen and close the pen, replacing the point.