You have the option to delete some of your assets (including their scan results) and add new ones instead. Or you can upgrade to a higher plan, which allows you to add more assets and keep your scan history.
From the /assets page you can delete one or several assets by selecting the checkbox right next to it and clicking Delete. Be careful though, deleting an asset will also delete all its associated scans.
You can also move one or several assets to another workspace by clicking the Move button.