First, start by unscrewing the thumbscrew by hand or with a Torx T6 bit.Then, unscrew the tip of the pen and let the refill and spring slide out along with the piston.Take the new piston with the hole with the threading facing up, and guide it back into the body of the pen until you see the hole where the thumbscrew should screw into.Take the thumbscrew and screw it into the piston and tighten it down until firm.Place the spring over the refill and screw the tip back on.