The main logo of a site is added from the theme editor. A secondary logo can be added from the site editor to co-brand or enhance brand recognition.
Add/remove secondary logo through the site editor
Add the secondary logo to your media library:
Click the Media Library icon from the left sidebar
Click the Upload Media button on the upper right
Select Upload File from the options and select the logo from your local machine
Click the Sites and Templates dashboard icon from the left sidebar
Hover over the target site and click the Edit button
In the site editing interface, click Edit on the left next to the site title
Select Display secondary logo.
Find and select the desired logo in the Media Library.
Click Done Editing.