Squeeze the sides together with needle nose pliers or a screwdriver. Then use a cotter pin puller or hook to pull out the pins from t he bottom of the piston.
Make sure your new pins are the correct diameter and length. Use a screwdriver to spread them apart.

Here’s what I found: They often have un-aligned holes in the piston boss and people use a smaller cotter pin in order to get it through the holes. So what I did was to get the wrist pin aligned with the hole you can see through the bottom of the piston and then use a long drill that fits the wrist pin (and going from memory I think it’s 5/32nds) and drill through. The piston hole and the wrist pin hole will align the drill as it passes through the other side. Then you can use the full size cotter pin. The undersize cotter pins will allow the wrist pin to rotate and eventually the cotter pin will break.