How can I delete a conversation if I no longer need it?

If you just want to restart your conversation you can set up in the Botpress dashboard -> integrations-> on the bottom, select "allow user to delete conversation" and then you save the changes and you should be able to clear the conversations by clicking on the new arrow

And I can't delete this one manually?

I dont know

If Bressbot is in the beta phase, you know that there aren't many functions.