You can click "Export products", follow the instructions,
change data in result file and import it again with checked
"Get columns names from the first row of CSV file"
and checked "Drop all products before import".
NOTE: Please backup your database before every import!
NOTE: You will loose all bestsellers, featured items, linked products, detailed images as well as all orders history.
** Consider upgrading to "Product Import II" module if you plan to update your products after your store is live.