how to delete a question ?

You cannot delete answered questions. Unanswered ones can be deleted by clicking on edit button, select Delete. Make Anonymous if you want to hide your name from question.

Remove a question

You can only remove a question before it receives its first answer.

Go to the Yahoo Answers page where you posted your question.
Click the Edit icon Edit icon.
Click Delete Question.
Click Delete to confirm

https://help.yahoo.com/kb/answers/remove-content-information-yahoo-sln25593.html

You can only delete a question if there is no answer. You must first click on the question to open a new page. There will be a blue box with the word "edit" with the "update", "make anonymous", and "delete" options.

You can only delete an answer if there is no best answer. There will be a blue box with the word "edit" with the "edit", "make anonymous", and "delete" options.

i don't think you can

You can't.

You can't anymore.