You can't edit a poll after it has been submitted. Since polls are more interactive than posts, I guess the theory is that if you are allowed to manipulate a poll it would make it invalid. I'm not sure how valid that explanation is though.
If you only need to add to or modify the poll items you can PM a moderator who can do that for you. You can't change a poll type, for example from single selection to multiple selection. If you need to do that, you would need to create a different poll.
(And there is no need to add the word "Poll" to the title, it will be in the title automatically.)