The Apple Pencil comes with a spare tip in the box. This can be used to replace the tip on your stylus when it wears out (we assume this is something that happens after you’ve used the Apple Pencil for a while, although it’s looking fine so far). And if you take the original tip off for some reason and then lose it, the spare will save your blushes.
Whatever your reasoning, you remove the tip by unscrewing it anti- or counter-clockwise. Underneath is a metal nib used to detect motion and pressure. Replace the Apple Pencil tip by screwing the new one (or the same one) back on, clockwise. Don’t even think about using the Apple Pencil without the tip attached (it’ll only scratch the screen).
We’re not sure how long the tips are supposed to last, but we’d guess it’s a fair while. You can’t currently buy further replacement tips on their own, so make sure you don’t lose the second one too.