Permalink is a permanent link address that is used to access a blog post. Blogger generates this address automatically for you by following a default pattern. Till date, Blogger never allowed the users to generate permalinks of their blog posts by their own but assigned a permalink automatically by reading the title of post.
For example, if the title of your blog is "My New Pictures" then the blogger automatically generates a permalink like;
http://blogname.blogspot.com/12/06/my-new-pictures.html.
Now, Blogger has given access to change the permalink however you can only change the bold part of above URL. You can not change the entire structure of your post URL. This feature is currently in draft and you can get this feature from http://draft.blogger.com. Simply click on New Post and you will see the permalink option on the right side of post editor.
This feature is only for your new posts, you can not edit or change permalink of your previous blog posts.