Go to Dashboard>Appearance>customize>Site Identity.
Logo: It changes the main logo of the site.
Site Title: It changes the main site title of the site.
Tagline: It changes the tagline (i.e. Description) of the site. Just below site title.
Site Icon: The Site Icon is used as a browser and app icon for your site. The icon must be square, and at least 512px wide and tall.
Set your site’s title, description and add a logo by navigating to Customizer > Site Identity.