how do I put my decapper pin back in my Lee full length die?

The neck sizer should be knurled. Carefully grab a hold of that with pliers and Take a screw driver in the other end of the decapping/neck sizer and loosen the knurled neck sizer.

This will loosen up its grip on the decapper pin and you can slide or pull it out. Place a new one in there and retighten and you are ready to go.

lee's have that slider thing you just loosen up the nut on the top and slide it back in and tighten the crap outta the nut they take a 9-1/16th's or 1/2 and a 3/4 to do this
you might wanna gring it up a bit rough before you put it back in.
or just get pissed and throw it out in the pasture like i did, and buy rcbs.or redding.

whelen - It is a full length die.

It doesn't matter they all have the neck sizer ball that the decapper mounts in.