Pencil tips are a consumable item - at should be replaced whenever you can see visible wear. Spare tips can be purchased from Apple in packs of four:
- Apple Pencil Tips - 4 pack
https://store.apple.com/uk/xc/product/MLUN2ZM/A
If you visit your local Apple Store or Apple Authorised Service Provider to buy the replacement tips, you have an ideal opportunity to have the technicians at the Genius Bar inspect and test your Pencil.
To replace the tip, carefully unscrew being careful to avoid damage to the fragile metal transducer that is otherwise covered by the tip. When replacing with a new tip, ensure that the tip is screwed firmly into place; when correctly positioned, you should see a tiny gap between the visible back of the tip and the body of the Pencil - the gap being about the thickness of a sheet of paper.