Your site title is used in your site’s header. If you upload a logo, your site title is replaced by the logo.
You can set the style properties (font / size / color) for your site title in the Design > Logo section.
To specify your site title, click on the design icon. This will take you to the design section.
In the design section, you can specify the Site Title as shown below.
Publish and you are done.
You can also upload you logo from the same page.
Click on Upload button to add your logo. Logo should be in JPEG / JPG / PNG format. Publish after you have uploaded the file.