Remove the screw from the shaft holding the spinner in place by turning it counterclockwise using a screwdriver. Remove the O-ring and remove the spinner.
Pull the old cutter line out of the spinner by sliding the spinner post out of the spinner and pulling the line out of the holes in the spinner post.
Slip one end of a replacement Flowtron cutter line, or its equivalent, through two of the holes in the spinner post. Put the ends of the cutter line together and adjust them so that the ends are even with each other. Repeat the procedure for the other line on the other side of the spinner post.
Feed both ends of one line through the corresponding ferrule hole in the spinner housing, then repeat the procedure for the line on the other side of the spinner post.
Slide the spinner post back into the spinner housing and slip the housing over the shaft. Replace the O-ring. Thread the previously removed screw clockwise into the shaft and tighten it using the screwdriver.