Step one: release the Friendly URL:
  1. Navigate to Content > Content Management System
  2. Hover over the website name and select Manage Website
  3. Click on the Content Management tab and switch to Folder View
  4. Under the Site Structure, navigate to the old content that has the Friendly URL assigned
  5. Click on the page to open it up
  6. Click on the Advanced tab
  7. Expand Link Options
  8. Beneath the Friendly URL field, click Release to free up the Friendly URL that is in use
  9. Click OK
Step two: assign the Friendly URL to a new page:
  1. Under the Site Structure, navigate to the new content to which you want to assign the Friendly URL
  2. Click on the page to open it up
  3. Click on the Advanced tab
  4. Expand Link Options
  5. Beneath the Friendly URL field, click Add
  6. Insert the desired Friendly URL and click OK
  7. Click OK to close out of the page
Also see: How to release a Friendly URL when the CMS content page has been deleted