How to Modify Permalinks on Blogger posts?

From Blogger product another feature is added i.e "Peramalinks" where user can access a blog post very easily. This is an auto-generated address available on blogger enabled users to create permalinks for their blog posts according the blog post title.
Let's take an example how its works If your blog title is "My blogger Tricks" then the blogger generates a permalink automatically as show in the below example

http://blogname.blogspot.com/12/07/my-blogger-tricks.html
Users can change only the above title ( My blogger Tricks ) URL permalink as above not entire structure of post Link.
All this you can get on http://draft.blogger.com as shown in the image below.

Note: Blogger not enabled this feature for Old posts only new posts have the provision to change permalink option.