You can remove an asset from your database by hitting the Edit Asset button on its Lansweeper webpage and then selecting Delete Asset.
You could also use your cleanup options to mass-remove assets. Your cleanup options can be found in the web console under Configuration\Scanning Setup\Server Options. Note that cleanup actions are performed when the Lansweeper service is started and subsequently every 24 hours. If you immediately wish to apply changes you made to your cleanup options, restart the Lansweeper service.
It is not currently possible to mass-delete assets by manually selecting them, as you could do in the Lansweeper\Scanned Computers section of the Lansweeper 4.2 configuration console. We will likely implement a feature like this when we're a bit further along in the beta and have fixed known bugs.