When you log in to OpenSolar for the first time or if you haven't uploaded a logo yet, you will see an Upload Logo button in the Header. You can directly upload the logo from any page (except Studio) by clicking this button.
Note: The upload logo button is not available on mobile screens.
To edit your company details including logo, you can go to Organisation Details section in the Control Page.
The video below will guide you through the whole process of setting up your Logo and Company Details.
See this Article for information on setting up your Company Information.