Stylus just has a little plastic carrier for the 5mm LED — so the shiny end pushes out the open end of the tube and the two wires go toward the other end; one wire wraps around the plastic carrier to make the positive connection, and the other wire rubs the metal tube for the negative. So you just push really firmly on the existing LED with batteries removed, and it and its plastic carrier will slide back into the battery tube and can be removed. Make sure to note the orientation of the LED plus and minus wires in the plastic carrier.

That’s all I remember from replacing LEDs in Stylus penlights many years ago. I think I had to tap the original LED with a mallet to get it to slide back into the battery tube, and press comparably hard with a pencil inside the battery tube, to get the new LED to slip into place.