[header] How to add lead to a mechanical pencil [title] Replace cassettes. [step] Exact instructions may vary by model, but generally, start by pulling the eraser out of the pencil, which will draw out the old cassette. Give the old cassette a shake just to make sure that it really is empty. 
If so, insert a new cassette into the pencil's open chamber. Once it clicks into place, reinsert the eraser after detaching the old cassette.