You can change your book’s slug by going here:
...making sure to replace YOUR_BOOK with your book's unique web address.
If your book is published, the old URL will redirect to the new URL. We do not redirect URLs when you change the URL for an unpublished book.
This change will not affect current readers of your book, i.e. existing readers will still get updates etc. as usual.
If your book is NOT published, then if you change the URL, the old URL will not redirect to the new URL.
You can also find the URL page under "Settings" on your book's Overview page: