All the Site Editor’s pages have the Build a Website title displayed next to favicons in browser tabs by default.
To change the Title of the Page:
- click PAGES in the left sidebar of the Site Editor:

- hover over the name of the page which title you would like to change and click the Settings button that appears to the right of it:

- enter the Title of the page into the TITLE field:

The Title of the page on the live website will change once you publish the site:
Change the Titles of the rest of the pages in Page Settings of Site Editor: