There are 2 ways on how to change the Site Title and Tagline of a site. You can either go to Appearance > Customize or Settings > General.
A. Go to Appearance > Customize and this page will open. Expand the Site Title & Tagline block to replace the info in the field. Click the Save and Publish button when done with your changes.
B. Another way is to go to Settings> General. In the General Settings page change the info in the field then click the Save Changes button at the bottom of the page.