Well Hi, Welcome to DP There are few buttons on the right hand bottom corner of your post, there is an edit button in that too. Just click on that an edit your post
You can edit your post during some time after posting using the [edit] button (I thinks it's 6 hours) after what, it's no longer possible to edit.
You can edit your original post within 8 hours of the posting time. If you still want to get it edited and its past that 8 hours limit, you can PM a mod and they will do it for you.