I am new to this site. Is it possible here to edit questions which already have answers?
Yes, it is possible to edit questions here, even if they already have answers. However:
Edits should always respect the intent of the original author.
Edits shouldn't change the question so much that existing answers become invalid.
Sometimes edits break these rules. When this happens, we can roll the edit back, restoring an older version of the question. If you have a specific example where an edit has caused problems, please feel free to flag the post for moderator attention.
So why do we allow edits at all? Because Stack Exchange is supposed to be a library of information for future users to learn from. We aren't just trying to help the person asking the question; we're also trying to help all the other people in the future who have the same question.
If a question is formatted poorly or confusing to read, it makes it harder for future users to read and understand the question. And if a question has the wrong tags, they might not be able to find it in the first place. Edits can solve all of these problems and make the site a better place.