First, pull off the top barrel of the pen by pulling it straight back. You may need to rotate it a bit to free it as it has a tight friction fit. Once you have the top barrel off, you’ll be able to see the top of the cartridge which is generally a black knurled plastic cylinder. Rotate it counter-clockwise to unscrew it from the holder and pull it out. Put in the new cartridge in reverse order, i.e. insert it into the holder, screw it in clockwise until it stops and slide the top barrel back on. Your pen is ready for use again!