First of all you go to the post whose permalink you want to change. After going to the post, click on the Revert to draft icon. As soon as you click on Revert to draft icon, you will see the option to edit your post permalink. Now change the permalink of your post, edit it and then click on Publish. By doing this the permalink of your post will change.
Note: If the option of edit permalink does not show after drafting the post, then reload the page.