Hold the ejector button down on the Poseidon™ pipettor and you will notice that the metal tip ejector is snapped into a black piece of plastic. With your thumb, push the metal tip ejector out and remove it. Turn the connecting nut to the pipette tip holder counter clockwise. Carefully remove the piston and take the connecting nut off the old tip holder. Place the piston into the new tip holder and gently push them back into the pipettor. While holding the piston and tip holder in place, slide the connecting nut through and tighten it by turning clockwise. Hold the ejector button down, slide the metal ejector through the end of the tip holder and snap it back into the black plastic.