Remove the old eraser from your Paper Mate plasma mechanical pencil. You should be able to pull it out easily with your fingers. If the eraser is too worn down to be removed with your fingers, carefully use a toothpick or other sharp narrow object to pry the eraser out of the pencil.
Place two or three new pieces of lead into the channel under the eraser. Use only full pieces of lead. Do not use small broken pieces, as they may clog the channel.
Push a replacement eraser on top of the channel. Press the eraser firmly in place.