Project explorer-> Project -> Navigation: responsive web (tab) -> authentication (section)(image above) -> click on show and modify that page
You can change the page when you only have anonymous users allowed in the application and then you can see Authentication tab in the navigation page and select the Sign-In page you want to configure
This section of the Advanced Page Building learning path explains two options for changing your login behavior. Depending on your use case and skills you can make a choice. The section continues explaining the step by step approach of adding anonymous access and styling your login page.