Add a new blog URL (3) as well as a few related keywords (4).
As soon as you add the new URL, the delete option will be displayed.
Click on the reb bin on the right to delete the URL you need to be removed.
A quick note:
If you have tied the URL of a specific blogpost and not the generic URL of your blog and you need to replace it, add first a "dummy" URL (for example "www.example.com") so as the system will let you delete the old blogspost URL and then add the generic blog URL.