Currently, it’s not possible for end-users to edit the index URL of a site that's already being checked for Quality Assurance or Accessibility in the platform. Please contact Siteimprove technical support and we’ll be happy to change the URL for you, given that you are an account owner.
It is possible for Account Owners to edit the index URL of a site that's being checked with the Response module, see "How do I edit the URL of a Response checkpoint?"
Note: It is possible for Account Owners to add a new site to your subscription. For further information, check out our interactive tutorial showing you how to add a new site to your Siteimprove subscription.