I recently posted a question on Askubuntu, I realized later that its answer is obvious and only needs to read the man page of a command.
Now I am not sure whether to delete it or let it or just close it.
After reviewing your question, what's obvious to you is not obvious to newbies who might be digging.
I'm not going to delete your question because it could have value to users looking for information such as that which you asked about. Even the 'stupid' questions sometimes have value!
Typically, if you wanted to delete your own post though, users should see a delete button underneath their answer they can use (unless it's answered):
If the question has an answer, though, and that answer is accepted or upvoted you might not be able to do this, so you'd need to flag the question for moderator attention to request deletion.