The flapper valve discs can be replaced by removing the 12 screws holding the top plate on ( you need to remove the cam and the timer housing first ). The flat springs pop out of the top of the valve discs. A pair of needle noses pliers helps with removing the springs ( and popping them back in place ), and also with pulling the valves out of the valve body. It is a fairly simple procedure.

PLEASE check carefully that the area where the valve disc "seat" is 100% clean, and be sure to Install the Discs into the Valve Body, and THEN lower the Top Plate on top. The Valve Discs must all be INSTALLED and SEATED first, for best possible results ( less chance of problems or leaks ).

The only other "tricky" part is to be careful putting the 12 screws back in. The screws are self tapping. I recommend turning them slowly in reverse until you feel them drop into  the thread groove, then alternate tightening them down. Do not over tighten, since you are screwing metal into plastic.