Yes, you can edit/delete a Topic/Discussion you have posted if there have been no replies directly to your message.
To delete a new topic, click Delete in the Actions panel to the right of the message.
To edit a new topic, click Edit in the Actions panel to the right of the message.
To edit/delete a reply to a topic, click on the links at the bottom of the reply.
Once someone else has replied directly* to your message the Edit/Delete option goes away. In the situation that you still need to delete your thread, please post in Forum Comments so a moderator can delete.
*If there are replies posted to other messages in the thread, even if posted after your reply, you will still be able to edit/delete. It is when someone replies directly to your message by clicking the Reply link on your message that the ability to edit/delete goes away. Any replies to any messages in a thread will remove the edit/delete options from the initial message.