Navigate to ADMIN > SITE > SITE LOGO.

The image assigned in your Admin area under the Site Logo field is used for the header logo.

It is recommended to use a .PNG or .JPG file for the logo.  A PNG file will allow for any transparency in your logo to show up.

Recommended: Fits within 225px x 50px

Horizontal logos are best inside the logo spaces of this theme.

Note: We recommended that you properly size your logo before uploading it into the site.

Resources:

Logo Help Article