Here is a close up of the cotter pin and cup plug. Note that the cup plug has the lip facing inward where you can't see it from outside. You may need to get a spare cotter pin if you can't reuse the one after taking it out.
To replace the high pressure relief spring, remove the cotter pin shown in the pictures above. Then punch a hole in the cup plug with a punch that is ground to a sharp point. I use a medium weight ball peen hammer and use a sharp blow so as to punch a hole in the cup plug and not push it deeper into the bore. I recommend using a punch to reduce the chance of getting chips in the oil pump relief valve bore. One chip can cause the valve to stick and the oil pump to fail. I don't recommend drilling because of the chips.
Then get a #8, #10, and #12 screw from the hardware store about 1" long with matching washers. Screw the screw into the hole in the cup plug at least 3 turns with the washer under the head of the screw. Then hold the oil pump in a vise and pry the cup plug out. After removing the cup plug, then tip the pump and let the pressure relief valve and spring drop into your hand. ***Note which way the valve goes in***.
Then place the new spring in the oil pressure relief valve bore and then install the relief valve after that.
Then install the cup plug with the lip facing inward making sure that the spring is properly seated in the relief valve and the cup plug. Be careful when installing the new cup plug not to push it in too far past the cotter pin holes.
Then install a cotter pin (reuse the old one or get a new one).
I like to mark an "H" on the outside of the pump with a paint marker afterwards so I know that I put the high pressure spring in it.
