Knowledge → Tools → Reset Knowledge Base removes indexed content by source: WordPress content and products, crawled pages, uploaded documents, or manual entries and Q&A. You choose which, and confirm by typing RESET.
Why not just delete the rows
Because the plugin also keeps a ledger of what it has already indexed. Deleting entries without clearing that ledger leaves the sync convinced everything is up to date, so a re-sync does nothing and you end up with an empty knowledge base that reports success.
Settings, conversations, leads and ratings are never touched by a reset.