Currently, there's no way to change the URL of a publication you've already uploaded to Issuu.
If you would like a new URL for a publication, you would have to change the file name on your computer, upload the publication as a new publication on Issuu, then delete the old publication with the old URL from your Issuu account (if you want to).
NOTE: Deleting the publication will also delete the statistics.
When uploading a publication to Issuu, the document name that will be displayed in the URL will be the same as the file's name on your computer. Therefore, before uploading a publication to Issuu, make sure the title of your publication is how you'd want it to appear in the Issuu URL.
If you are looking to change the title and description that is displayed below your publication on Issuu, check out this article on Editing a Publication Title and Description.
If you are looking to change your Issuu account's username/URL, please see Changing Account Username.