I asked a question, that has only one answer with a couple of upvotes. Now I´d like to state it in a better way, but more like a new question than like an edit. Should I delete the old (worse) question and withit the answer? Or should I just asked "twice"?
If you just want to improve the question an edit will be fine. It will also bring your question back to the front page giving it new visibility. You can leave a comment to the answerer to tell him you have added details (he might even be notified due to your edit but I'm not sure about that).
Anyway you need to have rights to delete your question in other case you can just vote to remove the post.
If it's a completely different question then yes ask a new question. It might be worth referring to the original question and explaining why it's different.
If it's not that different really then editing is the right way to go.
I think the change over is where an edit would make any existing answers wrong (or misleading at best). If that's the case go with the new question.