How do you open and refill a Parker duofold ball point pen?

To open and refill a Parker Duofold ballpoint pen, first unscrew the pen's barrel by turning it counterclockwise. Once the barrel is removed, pull out the used ink cartridge gently. Insert a new Parker ballpoint refill into the barrel until it clicks into place, then reattach the barrel by screwing it back on clockwise. Make sure the pen is functioning properly by pressing the top to extend the tip.