Currently, there's no way to change the URL of content you've already uploaded to Issuu.
If you would like a new URL for your content, you would have to change the file name on your computer, upload the content as a new document on Issuu, then delete the old content with the old URL from your Issuu account (if you want to).
NOTE: Deleting the content will also delete the statistics.
When uploading content 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 content to Issuu, make sure the title of your content 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 content on Issuu, check out this article on Editing Your Content Title and Description.
If you are looking to change your Issuu account's username/URL, please see Changing Account Username.